I've built a number of antennas with coax traps made from both RG-174
and RG-58. There are a couple of ways to connect the traps, with one
(sort of cross-connecting the ends) giving a more favorable L/C ratio
than the other. But after measuring the trap L, C, and Q and doing some
EZNEC modeling, I discovered that it's often hard, even with the RG-58
versions, to get low loss on all bands the antenna is intended for. The
loss might be low on the band the trap "traps", but relatively high
(typically a couple of dB) on a lower band. Or vice-versa, depending on
the combination of bands that the antenna is for. It depends on the
combination of bands that the antenna is for -- some are ok, some not so
good. The fact is that the effective inductor Q of coax traps made with
RG-58 or smaller coax is mediocre.

For a lot of applications, a couple of dB is an acceptable trade for the
simplicity of coax traps, but for some it's not. If it's not, I'd
recommend doing some modeling, or careful comparison to a non-trapped
antenna.

One other word of advice is to make sure you weatherproof the traps in a
way that prevents any water from getting between the turns. Water
between the turns can increase trap loss substantially.

Roy Lewallen, W7EL

funkbastler wrote:
On Wed, 06 Aug 2003 21:59:37 GMT, "Bill"
wrote:


Anybody tried to build this trap antenna yet?

http://www.nerc.com/~jdegood/coaxtrap/
http://members.shaw.ca/ve6yp/
http://members.fortunecity.com/xe1bef/hf-antennas.htm

Need some help on where to tune the traps for the bands of operation, and
pruning the connection wiring. Can this be done with out a major test
equipment investment?


I love those coax traps. About 20 years ago, I spent several hours
with a sweep generator and scalar network analyzer building a set
of traps for 40/20/15/10 meters. Then, I spent almost all day putting
up the antenna. The process is basically "put up one section, check
swr, drop antenna, trim antenna, raise it back up, and repeat until
the SWR is acceptable" (typical dipole scenario). Then, however, you
add a set of traps and the next section, and repeat the entire process
for the next lower (in frequency) band. Keep repeating till you have
all bands covered. Got a nice sunburn and a decent antenna out of the
deal.

The traps can be fairly light weight and compact - I used RG-174 and
wound 'em on empty pill bottles. For test equipment, a plain signal
generator and oscilloscope will suffice, or maybe even just a grid-dip
meter.... Heck - I even got so involved with coax traps that I wrote
a computer program to characterize the things and tell me how many
turns of what kind of coax on what size form would be resonant at
my frequency of interest.

Since that time, I've discovered how pointless it was. Like WB3FUP
suggests, just put up as much wire as you can, as high as you can.
Right now, I'm using a 130 foot inverted L, and it works every bit
as good as the trap dipole did (if not better) from 160 through
10 meters - and it was a *lot* easier to put up. The only drawback
is that you gotta have a tuner.

-fb-