Topband: 40/80/160 Meter Antenna

[Tom Rauch]

> 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 (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
W8JI@contesting.com