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[TowerTalk] SB-220 affecting SWR curves

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Subject: [TowerTalk] SB-220 affecting SWR curves
From: (lightfoots)
Date: Fri, 30 May 1997 01:35:59 -0700
Pete Smith wrote:

>Sorry to hit more than one reflector, but this one really seems to straddle
>I have an SB-220.  This morning, I measured the SWR curves of my Force 12
>C-3 through the SB-220 (with the amp off) and then with a barrel connector
>replacing the amp.  To my surprise, there was a huge difference!  For
>example, on 20m. with the amp in the line the SWR minimum was at 14.4 mHz,
>with very broad frequency response.  With the barrel, it was at 14.2 and
>the frequency response was much more like what one would expect.
>For the antenna folks - can you suggest a theoretical explanation?  It
>seems counter-intuitive that a foot of coax and two sets of relay contacts
>inside the SB-220 could have such a major effect.
>for the amp-heads - What should I be looking for inside the SB-220 that
>might explain this?  I'm a real neophyte with amps, and I don't want to go
>in any blinder than necessary.
>Thanks in advance.

There could be several reasons for this. The number one reason is usually a
poor directional coupler in the SWR meter. This is more common than you may
Another reason may be that there are reactances somwhere in the line,
causing the line to resonate with the whole system.

Try this:

Instead of putting the amp inline, just put another 3 foot piece of coax in it'
s place, then compare that reading with just going straigh to the antenna,
bypassing the 3 foot piece of coax.

You may be surprised what you see.

73 es CUL



Suggested Serving, Antennas not included.
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