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Re: [TowerTalk] Trap Resonance, field strength measurements

Subject: Re: [TowerTalk] Trap Resonance, field strength measurements
Date: Fri, 21 Nov 2003 17:10:45 +0100
List-post: <>
Hi Ron,

I can only second Dan's statements.

The variations you state (RS 59 to 55, and 10 or 20dB difference)
in my opinion sound _way_ too much for the differences I would expect
to be generated by changing a trap frequency.


At 07:44 21.11.2003 -0800, Dan wrote:
>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
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>TowerTalk mailing list
>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


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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