Topband: 40/80/160 Meter Antenna

[Jim]

Back in the July 92 issue of QST there was an article by Al Buxton, W8NX,
the described a space efficient 40/80/160 meter antenna. This antenna was a
trap design that used inner core of RG-58 coax wound on PVC pipe forms.

I want to build one of these antennas but the article was not clear in
exactly how to build the coils. If somebody has built one of these antennas
and would be willing to entertain some questions, I would appreciate it.

Thanks,

Jim, KC1RM

[Tom Rauch]

Post by Jim
Back in the July 92 issue of QST there was an article by Al Buxton,
W8NX, the described a space efficient 40/80/160 meter antenna. This
antenna was a trap design that used inner core of RG-58 coax wound on
PVC pipe forms.

Hi Jim,

I measured dozens of traps here a few years ago. Typical rankings
are:

Copper tubing and vacuum cap: 300,000 ohm Rp
60pF doorknob and #10 airdux: 250,000 ohm Rp
100pF doorknob and #12 airdux: 99,850 ohm Rp
Mosely TA33 79,000 ohm Rp
Cushcraft A3 76,270 ohm Rp
Coax RG-58/U 17,800 ohm Rp

Higher Rp indicates lower loss, you can see coaxial traps are very
poor. Even using expensive air-wound teflon insulated semi-rigid
copper tubing type coaxial cable, Rp was only 45,000 ohms. That's
because stubs, linear loading, and coaxial cable capacitors generally
have very low Q compared to other systems.

Marketing may have created other opinions, but that is how it all
works in real life. Traps aren't all that bad, unless they are
coaxial or linear-loading type traps.

Expect about 1.5dB or so loss for a coaxial trap in an Inverted L or
vertical at the trapped frequency. Loss using small airdux and a
doorknob would be less than 0.25 dB.

Loss where the trap is not "trapping" is insignificant with all
traps, so on 160 you should see little additional loss no matter what
trap you use.

Loss is **greatly reduced** and the antenna will work perfectly fine
if you set the trap resonant frequency 10-15% BELOW the band you are
trapping instead of in the band! Just readjust the antenna lengths to
move the SWR minimum and leave the trap OUTSIDE the band.

Tony Field (***@nucleus.com) had a program that calculates traps,
it was freeware. It is within 5% of the actual frequency results I
measured, so no further trimming is needed if you use that
program.73, Tom W8JI
***@contesting.com

[[e-mail] (by way of Bill Tippett )]

Jim:

I'm not familiar with the article that you are referencing but I use a trap
in my 160/80m Inv L. The vert leg is 60ft and the trap is placed at the top
of the vert section.

I wound my trap on a 4" piece of PVC and used RG213 coax. I used the
attached freeware program to calculate the dimensions of the trap and it
came out right on the money. Most of the antenna analyzers on the market can
be used to check the resonant frequency of the trap.

Let me know if I can be of any further assistance.

73
Gregg
W6IZT

Gregg Marco
1200 Manor Crest Ct.
Marietta GA. 30068
h 770 321-9159
c 678 642-3603

[[e-mail] (by way of Bill Tippett )]

Tom:

I am curious about your statement regarding additional loss as a result of
using a low Q trap. You stated that the loss at the non-trapped frequency,
in this case 160 meters would be insignificant regardless of the trap type
used. So I am assuming that the loss that you are referring to is the loss
in effective gain at the trapped frequency compared to a 1/4wl vertical
operating against the same ground at the trapped frequency. Is this correct?

73
W6IZT

[Tom Rauch]

Post by ***@aol.com (by way of Bill Tippett )
I am curious about your statement regarding additional loss as a
result of using a low Q trap. You stated that the loss at the
non-trapped frequency, in this case 160 meters would be insignificant
regardless of the trap type used. So I am assuming that the loss that
you are referring to is the loss in effective gain at the trapped
frequency compared to a 1/4wl vertical operating against the same
ground at the trapped frequency. Is this correct?

That's correct, and of course this also applies to trapped dipoles
and yagis as well as low band antennas.

The exception would be "traps" using very low C and high inductive
reactance, which may have significant loss at the pass frequency.
This type of "trap" would be the high inductance coils used to
isolate antenna sections while substantially reducing length. Those
"traps" or isolators can seriously degrade the "pass" frequency
performance, and people building 160/80 antennas should keep in mind
the 160 performance can be seriously degraded by using very large
inductors as "traps".

Of course any antenna is better than no antenna, so these are just
guidelines to shoot for.73, Tom W8JI
***@contesting.com