The past couple of days I've been reviewing literature on trap dipoles.
My goal is to build a Field Day dipole for 40, 20, 15, and 10 meters (the
club already has 80m covered). For simplicity of operation, I want to avoid
an outboard tuner. So I'm considering a balun fed dipole that can be
trimmed up at the band edges with a rig's built in tuner if necessary. I
also want to avoid the patterns that occur on the upper bands with a
longer wire (I have looked over W4RNL's 44 foot multibander with interest,
but the high impedance on 15m requires a ladderline feed and a tuner, so
I've discounted it for this project).

So, I'm left with two practical alternatives, a fan or a trap dipole.
From reading this group's archives and various web pages, I've come to the
conclusion that while the fan dipole is simple to construct it is
susceptible to element interaction and may require additional support or
care in element spacing and antenna erection. These points seem to weigh
against it for a quick and nearly fool-proof antenna for Field
Day/event/emergency use.

That leaves me considering a trapped dipole. In this category are traps
using an inductor and capacitor and those made from coaxial cable. W8JI
has documented just how poorly these traps will probably perform compared
to those made of discrete components. However, given their (apparently,
as I haven't tried to build any yet) low cost they may perform well enough.

More research has uncovered that suitable capacitors and inductors are
hard, if not impossible, to find and when procured the parts may cost as
much (more?) as the Reyco traps offered by Unadilla. Perhaps decent
performance does have its price and the price for three pairs Unadilla
traps is reasonable for the simplicity I desire.

I guess this is a long and rambling post to get to the point that the trap
has become sort of a historical device in modern amateur radio antenna
implementations except for commercial yagis. For the record, at my home
QTH I use a center fed Zepp style antenna with a tuner, so I'm not averse
to that concept, I just want to avoid it with this special project.

Despite the lack of literature, are there some trap projects that are easy
to build, but perform better than coaxial traps? I must admit that I'm
not to good at scratch building things as I'm better at fixing stuff or
following a pattern than spending a long time experimenting.

I know this is long, but I wanted to share my thoughts to show the track
of my thinking on this.

73, de Nate

--

"The optimist proclaims that we live in the best of all possible worlds,
the pessimist fears this is true."

On Sat, 06 Jan 2007 19:22:59 -0600, Nate Bargmann
wrote:

while the fan dipole is simple to construct it is
susceptible to element interaction and may require additional support or
care in element spacing and antenna erection.

Hi Nate,

No more than trapped dipoles and in real comparison, probably less. No
antenna is tuned and put up working on the first attempt unless it is
a 2M ground plane at 30 feet. ....you are not describing that.

Given your hemming and hawing about parts cost, and willingness to
suffer what Tom describes as poor performance from cheaper components
- what's the problem?

73's
Richard Clark, KB7QHC

On Sat, 06 Jan 2007 19:22:59 -0600, Nate Bargmann
wrote:

The past couple of days I've been reviewing literature on trap dipoles.
My goal is to build a Field Day dipole for 40, 20, 15, and 10 meters (the
club already has 80m covered).


73, de Nate

Just an idea -- maybe a half-size "g5rv" would do what you want?

bob
k5qwg

Nate Bargmann wrote:
So, I'm left with two practical alternatives, a fan or a trap dipole.
From reading this group's archives and various web pages, I've come to the
conclusion that while the fan dipole is simple to construct it is
susceptible to element interaction and may require additional support or
care in element spacing and antenna erection.

While there's bound to be *some* interraction, there are construction
techniques that make additional supports unnecessary...
TV ribbon cable or "window" ladder line can be used to get elements
two-at-a-time. IIRC, the latter type can be found w/ copperclad steel
conductors (for mechanical strength). To get more bands, you can add more
parallel lengths of either line by using cheap/easy-to-make looms. This
method has been described in many ARRL Antenna Book & Handbook.

That leaves me considering a trapped dipole. In this category are traps
using an inductor and capacitor and those made from coaxial cable. W8JI
has documented just how poorly these traps will probably perform compared
to those made of discrete components. However, given their (apparently,
as I haven't tried to build any yet) low cost they may perform well
enough.

I've not tried them but others report favorable results. I've seen online
Javascript calculators that will get you close. The biggest drawback IMO is
that there's no easy way to adjust the resonant frequency.

More research has uncovered that suitable capacitors and inductors are
hard, if not impossible, to find and when procured the parts may cost as
much (more?) as the Reyco traps offered by Unadilla. Perhaps decent
performance does have its price and the price for three pairs Unadilla
traps is reasonable for the simplicity I desire.

I've made trap dipoles (80/40m) from scratch. Sure, to keep losses lowest,
ready-made air inductors are preferred. But, Barker & Williamson or Air-Dux
inductors aren't cheap, so I've "rolled my own" using PVC plumbing parts as
forms/insulators. You'll find HV transmitting capacitors equally expensive.
But then, a short stub of coaxial cable works as a capacitor (~30pF/ft). If
you initially cut the stub longer than calculated, you can trim it up to the
operating frequency with a pair of wire cutters. Tip: to prevent arc-over
when using high power, trim *only* the outer jacket/shield. Some
noncorrosive RTV seals the coax when you're done. A grid-dip meter is a
handy tool for adjusting the traps... but not *absolutely* necessary.

73,
Bryan WA7PRC

Bob Miller wrote:
Just an idea -- maybe a half-size "g5rv" would do what you want?

Not for 15m.
--
73, Cecil http://www.w5dxp.com

On Sat, 06 Jan 2007 18:04:39 -0800, Richard Clark wrote:

Given your hemming and hawing about parts cost, and willingness to
suffer what Tom describes as poor performance from cheaper components
- what's the problem?

No real big problem, just feeling out the group for any other ideas of
trap construction that my Google searches haven't revealed. That's all.

I've found more on coaxial trap design including VE6YP's Coaxial Trap
program. I have studied W8JI's material more closely and I see that he
found that the traps have lower loss above the design frequency than at
it. This would suggest that a 10m trap might be better implemented at a
design frequency of 27.5 MHz than 28.4 MHz.

I think this is what I'll shoot for.

- Nate

--

"The optimist proclaims that we live in the best of all possible worlds,
the pessimist fears this is true."

Thanks Bryan, I appreciate the practical construction advice. It is
always welcome.

- Nate

--

"The optimist proclaims that we live in the best of all possible worlds,
the pessimist fears this is true."

On Sun, 07 Jan 2007 19:49:38 -0600, Nate Bargmann
wrote:

I have studied W8JI's material more closely and I see that he
found that the traps have lower loss above the design frequency than at
it. This would suggest that a 10m trap might be better implemented at a
design frequency of 27.5 MHz than 28.4 MHz.

I think this is what I'll shoot for.

Sorry Nate,

I don't follow the logic of this at all. Are you expecting the trap
to cut off all frequencies above 27.5 MHz?

73's
Richard Clark, KB7QHC

Nate Bargmann wrote:

Thanks Bryan, I appreciate the practical construction advice. It is
always welcome.


FWIW I've made multi-band trapped dipoles in almost the same way;
however for the feeder I used light[weight] co-ax [RG58} terminated in
a 1:1 balun wound on a toriodal ferrite [IIRC, Phillips 4C6 material].

For tuning the traps, I jigged up an SO259 socket on some paxolin
board. The connection from the SO259 center conductor went via a 28v
light-bulb [voltage not critical I guess, but it's what I had] to a
flying lead terminated in a croc-clip., likewise the return to the
SO259 body via a flying lead and croc-clip. The coil/ co-ax capacitor
parallel tuned circuit is thus connected, in series with the
light-bulb, to my TX [IC735, on lowest power]. The resonant frequency
is indicated by ther light-bulb going dim or out altogether. Tweak coil
spacing/ trim co-ax capacitor to suit. Crude but effective.
HTH
73s de
G4GCJ

On Sun, 07 Jan 2007 21:47:05 -0800, Richard Clark wrote:

On Sun, 07 Jan 2007 19:49:38 -0600, Nate Bargmann
wrote:

I have studied W8JI's material more closely and I see that he
found that the traps have lower loss above the design frequency than at
it. This would suggest that a 10m trap might be better implemented at a
design frequency of 27.5 MHz than 28.4 MHz.

I think this is what I'll shoot for.

Sorry Nate,

I don't follow the logic of this at all. Are you expecting the trap
to cut off all frequencies above 27.5 MHz?

This is what W8JI's site hints at. Is he wrong, or am I misunderstanding
him?

- Nate

--

"The optimist proclaims that we live in the best of all possible worlds,
the pessimist fears this is true."