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

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
--

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.

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

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
--

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

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

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
news

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?

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
--

wrote:
. . .
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.
. . .

How do you get quantitative data from that method? How many degrees per dB?

A more direct way is to hoist the antenna up, measure the field
strength, then replace it with a full size untrapped dipole, measure
again, and compare. (Then hoist up the first antenna again and make sure
you get the same reading as before.) The best way to measure the field
strength is either with a fixed signal source some distance away and a
step attenuator at the receiver, or by enlisting the help of a
neighboring ham with either a fixed signal at his end or a fixed signal
at your end and a step attenuator at his. I've used this latter method
to measure a shortened 40 meter dipole. However, it's important to make
sure the other station's antenna is polarized the same and that you both
have well-decoupled feedlines. You also have to watch out for sky wave
interference -- we saw enough in the afternoon on 40 meters to produce a
dB or so QSB -- at a distance of one mile! So it might be necessary to
average some readings over a short period of time.

Make sure you measure it on all bands of operation. As I've mentioned
before, it's common to have higher loss on bands other than the one
where the trap is resonant.

Over the years I've gained enough trust in modeling that I believe the
model results for trap dipole loss. In fact, I've also gained a very
healthy respect for the difficulty in making even simple RF
measurements, so I often tend to trust the model results more than
measured ones. Of course, the model results depend on accurate trap
impedance measurements. But I'm able to make those with reasonable accuracy.

Roy Lewallen, W7EL