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...

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...

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
--

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
--

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

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

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
--

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
--

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?

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
--