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Re: [TowerTalk] Trap Resonance

To: "Ron & Madhavi" <>,<>
Subject: Re: [TowerTalk] Trap Resonance
From: "Dan" <>
Date: Fri, 21 Nov 2003 07:44:23 -0800
List-post: <>
Hi Ron: The extreme variations in signal strength with your different
configurations are difficult to explain and might be due to one of the
1. Large variations in SWR cause your transceiver protective circuits to
reduce its power output when they become too large. You can check this by
measuring the power actually applied to the antenna and the SWR. I would
look at this first.
.2. Large variations in signal strength to your friends receiver can be
caused by variations in the propagation path at different frequencies.
Attempts to measure yagi directivity with local friends can give very
strange results. I have seen reflections off a local water tower giving very
strange patterns.
I would build a simple field strength meter and locate it several
wavelengths away to tell what is going on.
73., Dan, N5AR
----- Original Message -----
From: "Ron & Madhavi" <>
To: <>
Sent: Friday, November 21, 2003 5:04 AM
Subject: [TowerTalk] Trap Resonance

I recently built a VERY SUCCESSFUL vertical for 40m which was capacitive
loaded with a 2x2' brass welding rod top hat ! The overall length was 23' (a
reduction of 10') and helped me elevate the feed point and radials. I
regularly work into Japan and Australia with this vertical with 4 radials,
running 80 Watts.

Since I have only 1 anchor point, I have moved on and am presently trying to
build a 40/20 trap vertical (without the hat). Meantime, I read Tom Rauch's
article about coax traps. Besides speaking of how lossy they are, he states
that one should NEVER have coax traps resonant on the actual operating
frequency. His data suggests that such a trap results in a loss of 1.6db,
whilst a trap resonant at 13.5 Mhz reduces the loss to .3 db. I have read
similar advise at L.B Cebik's site.

I built a trap resonant at 13.5 Mhz and placed it 14.5 feet along the 33
foot 40m radiator. The loading effect of the 20m trap of course resulted in
the shortening of the 33' radiator to around 29' for resonance on 40. Test
with a local friend indicated 59 at 14.145 but a drastic drop to 55 at
14.340. The 40m signal had also dropped 12 db from a resonant 40m only
vertical. I next tried to get the trap close to 14.350 (before night fall!)
and ended up pruning too much and it dipped at 14.5 mhz. This resulted in
the 20m section being 16' long and the 40m resonance moved up to 7.3 Mhz. I
increased the length of the top, to bring resonance to 7.080 (freq I work
JA). The 20, reports now increased to 59+5 across the band and less drop
between band edges. The 40m signal also went up by 10db or so. This
indiactes that by moving to the higher side, I gained in excess of 20bd at

I am now left wondering where exactly the trap should dip? Tom suggests that
it should NOT be brought to resonance on and around the desired operating
freq. So, where do I tune the trap? I could place it at 14.2 but 14.2 would
be my desired operating freq!

This also proves that building coax traps blindly, with no dip meter or
analyser is a dangerous exercise! Coax cables these days come in various
specs and is no more made to standard specs (unless you buy a well known
brand). I use an Autek RF1 and see a nice dip when I put a coupling coil
over the trap.

I am no expert, but I keep learning and find that re-inventing the wheel can
be an experience!

Any advise from you enlightened folk would be MUCH appreciated.

Ron, 4S7RO/6Y5


See:  for "Self Supporting Towers", "Wireless
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See:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

TowerTalk mailing list

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