Need design info on coax traps for dipoles
 

Good afternoon...

Anybody know where I can find some formulas or design information for
designing coax traps for a dipole?

I've seen some articles on coax-trap antennas but the traps they use are
too big. I need to make some traps for 40 meters that are narrower (but,
longer is OK) than the ones I've seen in the articles.

(A thought I had was perhaps using RG-174 instead of RG-58 ... these will
be used at low power only, 10 watts max)

High Q and narrow bandwidth is OK. These traps only have to resonate at
one specific frequency.

Any thoughts or advice?

Thanks...

Need design info on coax traps for dipoles
 

Send me a valid email address and I'll send you a coax trap program written by
Reg Edwards [SK]

C. J. Clegg wrote:
Good afternoon...

Anybody know where I can find some formulas or design information for
designing coax traps for a dipole?

I've seen some articles on coax-trap antennas but the traps they use are
too big. I need to make some traps for 40 meters that are narrower (but,
longer is OK) than the ones I've seen in the articles.

(A thought I had was perhaps using RG-174 instead of RG-58 ... these will
be used at low power only, 10 watts max)

High Q and narrow bandwidth is OK. These traps only have to resonate at
one specific frequency.

Any thoughts or advice?

Thanks...

Need design info on coax traps for dipoles
 

On Sat, 11 Nov 2006 14:24:04 -0500, "C. J. Clegg"
wrote:


Good afternoon...

Anybody know where I can find some formulas or design information for
designing coax traps for a dipole?

I assume that to mean a parallel LC circuit where the L is mainly
formed by the inductance of the outer of a coax cable, and the
capacitance is mostly formed by the inner to outer of the coax cable.

The programs that I have seen assume that the impedance of a short o/c
transmission line stub is purely capacitive and has a constant
capacitance per unit length.

Both assumptions are wrong, which is why these traps have higher loss
and are wider band (both inextricably linked) than predicted by those
programs.


I've seen some articles on coax-trap antennas but the traps they use are
too big. I need to make some traps for 40 meters that are narrower (but,
longer is OK) than the ones I've seen in the articles.

(A thought I had was perhaps using RG-174 instead of RG-58 ... these will
be used at low power only, 10 watts max)

At the further expense of efficiency, if that matters.


High Q and narrow bandwidth is OK. These traps only have to resonate at
one specific frequency.


You could wind a traditional solenoid with thick copper, and use a
quality fixed capacitor. Short o/c stubs of lossy coax make for lossy
reactors.

Try the calculator at http://www.vk1od.net/tl/tllc.php to find the
input impedance of 3m of RG174 with a 100MOhm load at 7MHz... I get
1-j64, so it is a 64 ohm capacitive reactance with a D factor of 0.016
(or Q of 64). You should expect an order of magnitude or more better
performance from a quality fixed capacitor.

But they "work". The benefit of wider trap bandwidth and less critical
tuning are at the expense of radiated power.

Any thoughts or advice?

Thanks...

Owen
--

Need design info on coax traps for dipoles
 

On Sat, 11 Nov 2006 21:03:20 GMT, Owen Duffy wrote:


Try the calculator at http://www.vk1od.net/tl/tllc.php to find the
input impedance of 3m of RG174 with a 100MOhm load at 7MHz... I get
1-j64, so it is a 64 ohm capacitive reactance with a D factor of 0.016
(or Q of 64). You should expect an order of magnitude or more better
performance from a quality fixed capacitor.


The selection of 7MHz for the above example should not be taken as a
recommendation that traps be resonant at one of the frequencies of
"operation"... in fact it is usually better that they are not operated
at resonance.

I also had a quick look at Reg's COAXTRAP prog. On an trial for a trap
at 6MHz using 5.2mm dia line, it came up with a resonant impedance of
187kOhms which seems very high. I make the 1.8m of coax o/c stub
bringing around 34kOhms of shunt resistance alone, so resonance
impedance should be less than that, and more likely less than 20kOhm.

Programs tend to underestimate the losses of these traps.

Owen
--

Need design info on coax traps for dipoles
 

C.J.

You can design the antenna and the coils using Hamcalc. Hamcalc is a
collection of GWBASIC programs that run in a DOS box in Windows. It
includes over a hundred programs that do a variety of ham calculations.
The collection is free and is updated regularly. No changes to your
registry, source included and no spyware!!

Available from http://www.cq-amateur-radio.com/HamCalcem.html

Roger

Need design info on coax traps for dipoles
 

On Sat, 11 Nov 2006 22:07:14 GMT, Owen Duffy wrote:

Programs tend to underestimate the losses of these traps.

Owen,

What is your estimation of the loses? Will it be more than a dB?

Danny, K6MHE

Need design info on coax traps for dipoles
 

On Sat, 11 Nov 2006 16:19:48 -0800, Danny Richardson
wrote:

On Sat, 11 Nov 2006 22:07:14 GMT, Owen Duffy wrote:

Programs tend to underestimate the losses of these traps.

Owen,

What is your estimation of the loses? Will it be more than a dB?

Danny,

I recall modelling dipoles incorporating such traps with NEC some time
ago. My recollection is that the loss is of the order you suggest,
which might seem insignificant in the context of a path budget, if it
wasn't for the fact that it might about to a significant amount of
power to be dissipated in the trap, depending on the power level /
mode.

Operation at resonance exacerbates the situation.

My sentiment is not so much that the traps are a bad idea, but the
design tools that are around seem to take shortcuts and are
inconsistent. The design tools would make one think that the trap
designs are better than they really are.

I tried Reggie's COAXTRAP and compared the predicted inductance with
his SOLNOID3, and they differ by a factor of 4, and SOLNOID3 produces
the more believable result. I don't know why that would be so.

Owen
--

Need design info on coax traps for dipoles
 

For another data point, I made a number of coax traps using RG-58, and
measured the L, C, and Q. I don't have the numbers handy right now, but
using the measured values I looked at overall loss when used in antennas
for various combinations of bands. In most 3-band configurations, there
was one band where the loss was a couple of dB, and not necessarily the
band where the trap was resonant. Some 2-band configurations were ok,
some not.

On Field Day, I sometimes use a supplementary 3-band dipole arrangement
pointed N-S for California -- my main antennas point to the east. It
consists of a 40-15 trap dipole, with a 20 meter dipole drooping under
it as a separate inverted vee but connected to the same feedline. With
RG-174 traps, the loss is tolerable with the 40-15 combination, but not
with 40-20-15.

Trap loss when used in an antenna is a function of many factors, and
there's really no practical way to determine what it'll be except for
modeling. Of course you have to know the equivalent L, C, and R or Q.
You have to check it on all bands, since it might easily be ok on the
band where it's resonant but lossy on others. Way back in 1998 I did a
brief study of the effect of L/C ratio on trap loss, assuming a constant
trap Q. The results, along with the EZNEC models I used, are at
http://eznec.com/misc/ as traps.zip. I haven't looked at them for years,
and see that I used Laplace type loads, since this was done before EZNEC
had RLC type loads available. Modern EZNEC has a special "trap" type RLC
load with the R in series with L, and C in parallel with the
combination, and with the ability to make R vary realistically with
frequency. Nonetheless, EZNEC users might find the models useful, and
others might be interested in the results given in the accompanying text
file. Note that the Q I assumed for the study, 400, is probably better
than you'll see with a coax trap.

No matter what kind of trap you make, it's essential to keep water from
getting between the turns of the inductor or coax. Letting that happen
is guaranteed to trash the Q.

Roy Lewallen, W7EL

Owen Duffy wrote:
On Sat, 11 Nov 2006 16:19:48 -0800, Danny Richardson
wrote:

On Sat, 11 Nov 2006 22:07:14 GMT, Owen Duffy wrote:

Programs tend to underestimate the losses of these traps.
Owen,

What is your estimation of the loses? Will it be more than a dB?

Danny,

I recall modelling dipoles incorporating such traps with NEC some time
ago. My recollection is that the loss is of the order you suggest,
which might seem insignificant in the context of a path budget, if it
wasn't for the fact that it might about to a significant amount of
power to be dissipated in the trap, depending on the power level /
mode.

Operation at resonance exacerbates the situation.

My sentiment is not so much that the traps are a bad idea, but the
design tools that are around seem to take shortcuts and are
inconsistent. The design tools would make one think that the trap
designs are better than they really are.

I tried Reggie's COAXTRAP and compared the predicted inductance with
his SOLNOID3, and they differ by a factor of 4, and SOLNOID3 produces
the more believable result. I don't know why that would be so.

Owen
--

Need design info on coax traps for dipoles
 

If your mind is set on coax traps, ok, but you might consider
something different. W9INN uses what he calls a "resonactor" in his
trap dipoles and I think they are far better. Basically, they are just
a single layer coil which is carefully trimmed to resonate at the
desired frequency with its own self-capacitance. There is no other
capacitance needed. The advantages are two: Very low loss and very
high power handling capability. I have run 1500 watts of RTTY under
contest conditions for years into mine with no problems at all.
Conventional traps, whether L/C or coax would probably have given
problems my now with that power level and type of operation.

You do need a grid dip meter to tweak them, but that is true of other
traps too.

W9INN is now an SK, but his ideas live on.

Bill, W6WRT


------------ ORIGINAL MESSAGE ------------

On Sat, 11 Nov 2006 14:24:04 -0500, "C. J. Clegg"
wrote:

Anybody know where I can find some formulas or design information for
designing coax traps for a dipole?

Need design info on coax traps for dipoles
 

On Sat, 11 Nov 2006 18:04:47 -0800, Roy Lewallen
wrote:


Trap loss when used in an antenna is a function of many factors, and
there's really no practical way to determine what it'll be except for
modeling. Of course you have to know the equivalent L, C, and R or Q.

Roy,

Sure. I should have mentioned that after trying NEC models to get a
low VSWR solution for just two bands (one twice the frequency of the
other) I gave up figuring that it was likely that an ATU would be
needed in any case, and the traps were just an unnecessary
complication.

As you know, trapped antennas are sometimes explained by considering
the traps to be equivalent to frequency selective switches that are
o/c or s/c at certain frequencies of operation, and conveniently
disregarding the conductors on the outboard side of the o/c switches.
It is a simplistic view, it does not lead to a design that works, nor
does it explain designs that do work, but it remains the most popular
explanation to be heard on air in my experience.

Owen
--

Need design info on coax traps for dipoles
 

Bill Turner wrote:
If your mind is set on coax traps, ok, but you might consider
something different. W9INN uses what he calls a "resonactor" in his
trap dipoles and I think they are far better. Basically, they are just
a single layer coil which is carefully trimmed to resonate at the
desired frequency with its own self-capacitance. There is no other
capacitance needed. The advantages are two: Very low loss and very
high power handling capability.

Consider that is a very good description of a resonant
coiled coax choke.
--
73, Cecil http://www.w5dxp.com

Need design info on coax traps for dipoles
 

Owen Duffy wrote:
As you know, trapped antennas are sometimes explained by considering
the traps to be equivalent to frequency selective switches that are
o/c or s/c at certain frequencies of operation, and conveniently
disregarding the conductors on the outboard side of the o/c switches.

Traps are high-impedance devices at their resonant frequency.
For them to work properly, they need to see a low impedance on
the far side of the trap. If they see a high impedance on the
far side, they cease to be "traps" and become the phase shifting
coils described by Kraus useful in a phased array.
--
73, Cecil http://www.w5dxp.com

Need design info on coax traps for dipoles
 

Owen Duffy wrote:

Sure. I should have mentioned that after trying NEC models to get a
low VSWR solution for just two bands (one twice the frequency of the
other) I gave up figuring that it was likely that an ATU would be
needed in any case, and the traps were just an unnecessary
complication.

As you know, trapped antennas are sometimes explained by considering
the traps to be equivalent to frequency selective switches that are
o/c or s/c at certain frequencies of operation, and conveniently
disregarding the conductors on the outboard side of the o/c switches.
It is a simplistic view, it does not lead to a design that works, nor
does it explain designs that do work, but it remains the most popular
explanation to be heard on air in my experience.

Yes, I'm afraid that's true.

I try to avoid simplified explanations just because people forget that's
what they are, and begin treating them like fact. A good example is the
recent thread about there being two "theories" of ground plane
operation, one being the simplified explanation of operation as an
"image". (I still remember an April magazine article in which the author
began with the popular and common "image" idea to explain the operation
of a grounded vertical. Then he proposed drilling a hole in the ground
and putting the vertical there, fed at ground level. This, he said,
resulted in the "image" being above the ground. The perfect stealth
antenna! Anyone who knows where this appeared -- must have been around
the '60s -- would do me a favor by letting me know. I'd love to read it
again. I've searched the entire collections of QST and HR to no avail.)

But back to traps. Trap operation is indeed a lot more complicated than
most people realize. I was recently contacted by a puzzled EZNEC
customer who saw a situation where a trap had very little effect. He had
put the traps where the current was normally very low, so of course the
traps did little. Plenty of current was being induced in the wire beyond
the trap by mutual coupling from the field produced by current on the
wire on inside of the trap, whether the trap was there or not.

The simplified idea of a trap as a short circuit on other bands also
leads to bad decisions. On any frequency band below its resonant
frequency, it acts like a loading inductor; on bands above resonance,
like a capacitor. It's common to see more loss on the lower frequency
bands where the trap is a loss inductor than at the trap's resonant band.

Modeling makes it easy to find out what works well and what doesn't for
a given application. But of course, you have to know the trap's L, C, and R.

Roy Lewallen, W7EL

Need design info on coax traps for dipoles
 

On Sat, 11 Nov 2006 15:11:14 -0800, sailtamarack wrote:

You can design the antenna and the coils using Hamcalc.

Thanks for pointing me in that direction. That program is a goldmine!

Also thanks to everyone else for their advice.

I looked at rfparts.com for suitable capacitors for these traps
(assuming I don't go the coax trap route), and I gotta say, they sure are
proud of their capacitors ... $16+ for each 5 KV doorknob capacitor.

For a dual-band 80m/40m dipole using resonant traps, how can I figure out
the capacitor voltage rating I need for each power level?

This antenna is never going to be used over 200 watts and rarely over 100
watts, in fact it's probably going to spend most of its life around 10
watts or less (FT-817 / IC-703).

Need design info on coax traps for dipoles
 

You guys sure are way beyond me in antenna and trap theory. :-)

I probably neglected to mention earlier, but this antenna is for portable
operation in the field on 75 and 40 (none of the other bands are needed),
so it needs to be easy to put up and take down and easily transportable,
also fairly rugged so things like airwound coils waving in the breexe with
ceramic insulators inside the coil probably won't work; instead the coils
will probably have to be tightly wound around small lengths of PVC pipe
and lacquered in to place.

I need to build several of these antennas (at least 10, I think, perhaps
more), and so $16+ each for doorknob capacitors from places like
rfparts.com isn't likely to be practical. That reason, along with ease of
construction (and now ease of design with programs like Hamcalc around) is
why I have been leaning towards coax traps.

The bandwidth (of the traps or of the antenna itself) is not important and
so high Q and narrow bandwidth is OK.

Power levels will be low, never more than 200 watts and probably never
more than 100 watts, and often around 10 watts.

If you guys say coax traps are a sub-optimal approach I'll take your word
for it.

But, the things you all are saying about traps coming with a lot of
baggage that no one ever thought of is making me nervous. :-)

It's not exactly a new concept, trap antennas have been used with great
success for what, 80+ years now?

If I forget the coax trap idea and go with a resonant coil-and-capacitor
approach on, say, a 1.5" coil form (which Hamcalc seems reasonably happy
with after warning me that 2" was too big and 1" was too small), what
should I look out for?

Need design info on coax traps for dipoles
 

On Sun, 12 Nov 2006 22:05:38 -0500, "C. J. Clegg"
wrote:


You guys sure are way beyond me in antenna and trap theory. :-)

I probably neglected to mention earlier, but this antenna is for portable
operation in the field on 75 and 40 (none of the other bands are needed),
so it needs to be easy to put up and take down and easily transportable,
also fairly rugged so things like airwound coils waving in the breexe with
ceramic insulators inside the coil probably won't work; instead the coils
will probably have to be tightly wound around small lengths of PVC pipe
and lacquered in to place.

I need to build several of these antennas (at least 10, I think, perhaps
more), and so $16+ each for doorknob capacitors from places like
rfparts.com isn't likely to be practical. That reason, along with ease of
construction (and now ease of design with programs like Hamcalc around) is
why I have been leaning towards coax traps.

The bandwidth (of the traps or of the antenna itself) is not important and
so high Q and narrow bandwidth is OK.

Power levels will be low, never more than 200 watts and probably never
more than 100 watts, and often around 10 watts.

If you guys say coax traps are a sub-optimal approach I'll take your word
for it.

But, the things you all are saying about traps coming with a lot of
baggage that no one ever thought of is making me nervous. :-)

It's not exactly a new concept, trap antennas have been used with great
success for what, 80+ years now?

If I forget the coax trap idea and go with a resonant coil-and-capacitor
approach on, say, a 1.5" coil form (which Hamcalc seems reasonably happy
with after warning me that 2" was too big and 1" was too small), what
should I look out for?


When you make your mind up about the MAXIMUM operating power level,
you will be in a better position to finalise the design. Capacitors
are likely to fail if you exceed their voltage at all, not for 90% of
the time, or 10% of the time, but at all, and in microseconds.

If you design a 75/40m trapped antenna, and place the trap resonance
away from 40m band (in fact any other operating frequencies), you will
reduce the voltage impressed on the trap. At low power levels, you may
well be able to use a capacitor or stack rated for around 1000V which
will come a lot cheaper and a lot lighter than a doorknob.

But, as Roy has told you, they don't fall into place. The question is
whether "hamcalc" will allow you to design with sufficient freedom and
rate the operating voltage on the traps. The technique of modelling
the proposed design will reveal the voltage expected across the traps,
so long as your estimates of the trap parameters are sufficiently
accurate.

It might be safer to copy a published design rather than looking for
design tools that might not be rock solid.

I cannot offer you a proven design, the thing that I use is pretty low
tech, it is an 80m dipole with insulators in the middle of each leg
and wander leads to switch between a 40m half wave and 80m half wave.

Owen
--

Need design info on coax traps for dipoles
 

C. J. Clegg wrote:
. . .
If you guys say coax traps are a sub-optimal approach I'll take your word
for it.

But, the things you all are saying about traps coming with a lot of
baggage that no one ever thought of is making me nervous. :-)

It's not exactly a new concept, trap antennas have been used with great
success for what, 80+ years now?

The coax trap is a relatively new idea. I don't think it was around
until the '70s. They definitely have their place, being cheap, simple,
and rugged. And the loss is minimal in a lot of cases. Note, however,
that there are two ways of connecting the coax. In one, the center of
one end of the coax is left open. In the other, the shield of one end is
connected to the center conductor of the other, and the two remaining
ends are the external connections to the trap. The latter method gives
an L/C ratio that's generally more favorable.

As for the "great success", almost no amateur is able to tell when he's
getting a couple of dB loss, which is about what you typically get with
an unfortunate trap/band combination. I've mentioned that this is what
happens and how I've avoided it, but most people happily accept it, if
it happens, without worry. Each to his own.

If I forget the coax trap idea and go with a resonant coil-and-capacitor
approach on, say, a 1.5" coil form (which Hamcalc seems reasonably happy
with after warning me that 2" was too big and 1" was too small), what
should I look out for?

For your application, I'd probably use coax traps. Using a separate coil
and capacitor aren't a guarantee against loss. Even a pretty good trap
can introduce a fair amount of loss if it ends up looking like a big
inductor in the wrong place in an antenna on a band below the trap's
resonant band. Or if you put it in a point where there's an extreme high
voltage between the ends on the band where it's resonant. Without the
ability to measure trap characteristics and do some modeling, what you
end up with will be determined entirely by luck. But as I said, you
probably won't notice the difference anyway.

Roy Lewallen, W7EL

Need design info on coax traps for dipoles
 

Roy Lewallen wrote:
The simplified idea of a trap as a short circuit on other bands also
leads to bad decisions. On any frequency band below its resonant
frequency, it acts like a loading inductor; on bands above resonance,
like a capacitor.

Until it gets to the 1/2WL self-resonant frequency where
it yields the 180 degree phase shift described by Kraus.

Quoting Kraus: "A coil can also act as a 180 degree
phase shifter ... The coil may also be thought of as
a coiled-up 1/2WL element." This coil is placed at a
current minimum point in a phased array.

This does NOT mean that the length of the wire used in
the coil is 1/2 wavelength. It just means the coil is
series resonant with a 180 degree phase shift.
--
73, Cecil http://www.w5dxp.com

Need design info on coax traps for dipoles
 

C. J. Clegg wrote:
For a dual-band 80m/40m dipole using resonant traps, how can I figure out
the capacitor voltage rating I need for each power level?

For a rough approximation, you can model the trap as
a stub with the same VF and Z0 as the coil. Equations
(32) and (51) in the following IEEE paper are useful
for that purpose.

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf
--
73, Cecil http://www.w5dxp.com

Need design info on coax traps for dipoles
 

I cannot offer you a proven design, the thing that I use is pretty low
tech, it is an 80m dipole with insulators in the middle of each leg
and wander leads to switch between a 40m half wave and 80m half wave.

Owen
Please describe "wander leads".

John Ferrell W8CCW

Need design info on coax traps for dipoles
 

C. J. Clegg wrote:
If you guys say coax traps are a sub-optimal approach I'll take your word
for it.

If inexpensive traps are the ultimate goal and the coax
is free from the local cable TV company, self-resonant
coaxial traps might be the way to go. How much loss is
in a coaxial trap?
--
73, Cecil http://www.w5dxp.com

Need design info on coax traps for dipoles
 

On Mon, 13 Nov 2006 13:40:16 GMT, John Ferrell
wrote:



I cannot offer you a proven design, the thing that I use is pretty low
tech, it is an 80m dipole with insulators in the middle of each leg
and wander leads to switch between a 40m half wave and 80m half wave.

Owen
Please describe "wander leads".

Sure, it is just 6 inches of wire with a crocodile clip on one end and
the other end wrapped+soldered to the wire on one side of the
insulator. The croc clips are either clipped to wire on one side or
other of the insulators to make a 40m half wave or 80m half wave.

Owen

John Ferrell W8CCW
--

Need design info on coax traps for dipoles
 

On Sun, 12 Nov 2006 01:29:20 GMT, Owen Duffy wrote:

On Sat, 11 Nov 2006 16:19:48 -0800, Danny Richardson
wrote:

On Sat, 11 Nov 2006 22:07:14 GMT, Owen Duffy wrote:

Programs tend to underestimate the losses of these traps.

Owen,

What is your estimation of the loses? Will it be more than a dB?

Danny,

I recall modelling dipoles incorporating such traps with NEC some time
ago. My recollection is that the loss is of the order you suggest,
which might seem insignificant in the context of a path budget, if it
wasn't for the fact that it might about to a significant amount of
power to be dissipated in the trap, depending on the power level /
mode.

Operation at resonance exacerbates the situation.

My sentiment is not so much that the traps are a bad idea, but the
design tools that are around seem to take shortcuts and are
inconsistent. The design tools would make one think that the trap
designs are better than they really are.

Unfortunately, a lot of ham lore also suggests that traps are much
worse than they actually are. The typical proponent of feeding a
"G5RV" with ladderline-balun-tuner rather than dreaming of using
traps probably believes that he has the more efficient system.

IMHO, for the case in question; 80 and 40 M operation with BW not an
issue, I would use coax traps and be done with it.

The major objection to traps in my estimation is the reduced BW that
results from the wire shortening and inductive loading on the lower
bands, not the additional loss. Any of the single feedline multiband
antenna *systems* that I can think of have higher loss than a
single-band resonant dipole. You can have your loss in traps or stubs
or you can have it in the transmission line/balun/tuner. Your choice.

I have placed a couple of files here that might be useful to the OP:

http://k6mhe.com/n7ws/QST_Correspondence_3_84.pdf

http://k6mhe.com/n7ws/QST_Correspondence_8_85.pdf

Also, Owen, I think you said earlier that the coax capacitance isn't a
linear function of line length because of transmission line effects.

The referenced authors have demonstrated that because of mutual
coupling between the coax conductors and the way the trap is
configured the line is actually just a capacitor.

Need design info on coax traps for dipoles
 

Roy Lewallen wrote:

of a grounded vertical. Then he proposed drilling a hole in the ground
and putting the vertical there, fed at ground level. This, he said,
resulted in the "image" being above the ground. The perfect stealth
antenna! Anyone who knows where this appeared -- must have been around
the '60s -- would do me a favor by letting me know. I'd love to read it
again. I've searched the entire collections of QST and HR to no avail.)

I remember an article with a slightly different twist on it. The
article proposed a use for old oil drilling sites - reusing the pipe
that was left behind in the hole. I think that it may have been a
Contest Journal April issue around 10 or 15 years ago.

tom
K0TAR

Need design info on coax traps for dipoles
 

On Mon, 13 Nov 2006 16:10:04 -0700, Wes Stewart
wrote:

Also, Owen, I think you said earlier that the coax capacitance isn't a
linear function of line length because of transmission line effects.

The referenced authors have demonstrated that because of mutual
coupling between the coax conductors and the way the trap is
configured the line is actually just a capacitor.

Hi Wes,

I was referring to the configuration where the coax forms a o/c stub.

I know that there is an alternative configuration that is claimed to
be superior.

Looking at that alternative configuration where the outer of one end
of the coil is connected to the inner of the other end, and the
external terminals of the trap are the remaining ends:

One way of viewing the circuit is that it is an inductor formed by the
outer conductor of the coax connected to a peice of transmission line.
The impedance as transformed by the transmission line appears in
series with the inductor between the trap terminals.

Ignoring the interturn capacitance of the inductor, and using a trap
resonant at 7MHz using Belden 8262 (RG58C/U) as designed by VE6YP the
circuit looks like an inductance of 3.344uH with a reactance of 147.07
ohms. Lets assume a Q of ~200, so assign a series R of 0.7 ohms.

The coil is 0.7+j147.

VE6YP suggests the length of coax required is 1.53m.

Using my line loss calculator to determine the input Z of 1.53m of
8262 with a 0.7+j147 load gives input Z of 830-j3936.

Reducing frequency in search of the impedance maximum, I find it at
6.870MHz with inductance now 0.7+j144.3 around 20k ohms. This
impedance would appear in series with that of the inductor, so the
trap would have a Z somewhere around 20k ohms. (The Z is very
sensitive to the assumed resistance of the coil, and the loss in the
transmission line.)

Is there a flaw in this approach?

I am a little suspicious of an explanation that says the transmission
line does not act like a transmission line.

Coincidentally, the input impedance of a 1.53m o/c stub of that line
at 6.87MHz is 0.52-j144.3 (or exactly the opposite reactance to the
inductor). A 0.7+j144.3 inductor in parallel with a 0.52-j144.3
capacitor yeilds Z of ~17k ohms... not very different to the superior
configuration... if my explanation / analysis is valid.

Owen
--

Need design info on coax traps for dipoles
 

On Mon, 13 Nov 2006 20:21:45 GMT, Owen Duffy wrote:


Please describe "wander leads".

Sure, it is just 6 inches of wire with a crocodile clip on one end and
the other end wrapped+soldered to the wire on one side of the
insulator. The croc clips are either clipped to wire on one side or
other of the insulators to make a 40m half wave or 80m half wave.

Owen

John Ferrell W8CCW
I was hoping you had a method that allowed the change without dropping
the antenna!

John Ferrell W8CCW

Need design info on coax traps for dipoles
 

C. J. Clegg wrote:
Good afternoon...

Anybody know where I can find some formulas or design information for
designing coax traps for a dipole?

Reg Edwards' COAXTRAP will get you close.

However, in my experience (several antennas since 1985, moslty 80/40
coax-trap dipoles and inverted Vees) with such traps, the formulas are
only a guide. Unless you are very very lucky, you'll need to check them
with a grid-dip meter. Adjusting resonance isn't easy, either. Be sure
to check them with nothing attached.

Any thoughts or advice?

Here ya go:

1) Use only high quality solid-dielectric coax. RG-58 is better than
RG-174. Bigger pipe is better than smaller pipe - I use 2". Be ready to
make a few test traps and waste some coax and PVC pipe.

2) Be aware that coax traps are not symmetrical. Which way you connect
them in the antenna matters. (I learned this the hard way). I
standardized on "center conductor end of trap goes towards center of
dipole".

3) Weatherproof with plastic spray or some such. Even so, the traps'
useful life is a few years. They're cheap, make a bunch.

4) You will have to test the traps themselves with a dip meter, and the
antenna itself with an SWR meter. I've been able to get my antennas
right where I want 'em this way, but it takes a bit of patience and
keeping good notes. Some Burndy split-bolt connectors help, too.

5) Some folks claim lower losses by using two pieces of coax in
parallel. I haven't tried this, because it's even more work than making
a single-coax trap.

6) I have seen all sorts of claims for trap losses, but I don't know of
any actual tests done to measure the real-world loss when used in an
actual antenna. Such a real-world test might consist of building a test
dipole that could be quickly lowered, running the legal limit into it
for a measured period of time, then measuring the temperature rise.

7) I do know that in side-by-side operations on Field Day, stations
using my homemade coaxtrap dipoles consistently equalled or
outperformed stations using Carolina Windoms and G5RVs on the same
bands. (Same power level of rigs, etc.)

8) Consider twin-traps. Reg Edwards has a program for them, too.

A TwinTrap uses the coax trap idea applied to parallel line. The trap
consists of a bifilar winding of wire on a piece of PVC pipe, with the
End of Wire #1 connected to the Start of Wire #2. Start of Wire #1 and
End of Wire #2 go to the antenna wires. The formulas say a TwinTrap has
more L and less C than a typical coax trap, which may or may not help
you. They are certainly easier to adjust!

--

My current antenna is an 80/40 coax-trap inverted V with the center at
about 37 feet up and the ends about 12 feet up. Traps are RG-58 on 2"
pipe (actual diameter about 2-1/2" IIRC), fed with RG-8X, no balun.
Adjusted for minimum (less than 1.2 to 1) SWR at 3570 and 7070 kHz,
will work on 20 with a tuner and about 3:1 SWR.

This antenna has been up several years at this QTH and wasn't new when
I put it up. Should be replaced next summer. I use it with my homebrew
100 watt CW transceiver (google my callsign under "web" to see the
station.)

In each of the past three November CW Sweepstakes, I have made over 420
QSOs per year, and at least 74 of the 80 sections, from here in EPA.
Best was 443 QSOs and 77 sections. Never spent more than 19 hours in
the contest. One of these days I may go to computer logging.

Good luck!

73 de Jim, N2EY

Need design info on coax traps for dipoles
 

The link to VE6YP's page seems to have changed to this:
http://www.qsl.net/ve6yp/

Have fun making your traps.

--
Pete . .
ZS5ACT / ZS5ZZ


------ Reply Separator ------

"C. J. Clegg" wrote in message
...

You guys sure are way beyond me in antenna and trap theory. :-)

I probably neglected to mention earlier, but this antenna is for portable
operation in the field on 75 and 40 (none of the other bands are needed),
so it needs to be easy to put up and take down and easily transportable,
also fairly rugged so things like airwound coils waving in the breexe with
ceramic insulators inside the coil probably won't work; instead the coils
will probably have to be tightly wound around small lengths of PVC pipe
and lacquered in to place.

I need to build several of these antennas (at least 10, I think, perhaps
more), and so $16+ each for doorknob capacitors from places like
rfparts.com isn't likely to be practical. That reason, along with ease of
construction (and now ease of design with programs like Hamcalc around) is
why I have been leaning towards coax traps.

The bandwidth (of the traps or of the antenna itself) is not important and
so high Q and narrow bandwidth is OK.

Power levels will be low, never more than 200 watts and probably never
more than 100 watts, and often around 10 watts.

If you guys say coax traps are a sub-optimal approach I'll take your word
for it.

But, the things you all are saying about traps coming with a lot of
baggage that no one ever thought of is making me nervous. :-)

It's not exactly a new concept, trap antennas have been used with great
success for what, 80+ years now?

If I forget the coax trap idea and go with a resonant coil-and-capacitor
approach on, say, a 1.5" coil form (which Hamcalc seems reasonably happy
with after warning me that 2" was too big and 1" was too small), what
should I look out for?

Need design info on coax traps for dipoles
 

On Tue, 14 Nov 2006 13:00:44 GMT, John Ferrell
wrote:

On Mon, 13 Nov 2006 20:21:45 GMT, Owen Duffy wrote:


Please describe "wander leads".

Sure, it is just 6 inches of wire with a crocodile clip on one end and
the other end wrapped+soldered to the wire on one side of the
insulator. The croc clips are either clipped to wire on one side or
other of the insulators to make a 40m half wave or 80m half wave.

Owen

John Ferrell W8CCW
I was hoping you had a method that allowed the change without dropping
the antenna!

No, no magic there John.

I was referring to use of a antenna for field operations, and mostly I
would rig the antenna as a sloper or an inverted V and it was
relatively easy to lower it, albeit inconvenient if weather was
poort... but field operations are not about convenience, now are they!

Owen
--

Need design info on coax traps for dipoles
 

Need design info on coax traps for dipoles
 

On Tue, 14 Nov 2006 08:25:17 GMT, Owen Duffy wrote:

On Mon, 13 Nov 2006 16:10:04 -0700, Wes Stewart
wrote:


Reducing frequency in search of the impedance maximum, I find it at
6.870MHz with inductance now 0.7+j144.3 around 20k ohms. This
impedance would appear in series with that of the inductor, so the
trap would have a Z somewhere around 20k ohms. (The Z is very
sensitive to the assumed resistance of the coil, and the loss in the
transmission line.)

Is there a flaw in this approach?

I think there is, the voltage drop across the coil will be very
similar to that looking into the coax, the the total voltage drop
across the whole trap might be more like double, and the impedance
more like double. I need to develop a more detailed model to get
better accuracy!

Owen
--