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

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."

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