----- Original Message -----
From: "N2TK, Tony" <email@example.com>
To: <firstname.lastname@example.org>; <email@example.com>
Sent: Friday, January 06, 2006 2:22 PM
Subject: Re: [TowerTalk] Yagi Separation on a Mast
> I had put up the C31 first and checked resonance. Then I put up the
> by itself to check resonance. Then I put up both together and there
> were no
> changes. By the way the booms are in parallel. I seem to still be
> competitive on 15M. Don't have a direct way to compare 15M to
> another beam
> to make sure no interaction. My other tribander is down the tower at
> 42' so
> no way to use that as a real reference.
The easiest way to check for interaction on a directive antenna is to
measure its f/b ratio. The f/b ratio is a very sensitive indicator of
interaction. SWR, gain, and resonant frequency are very insensitive
Here is the easiest way to do a f/b measurement:
1. Have a local ham (not in the near field) transmit a signal.
2. Point your antenna away from the local ham.
3. Record your s-meter reading.
3. Point your antenna toward the local ham.
4. Crank in fixed attenuators until your s-meter goes back to the
5. Read your f/b ratio off the attenuator setting.
You can use an external attenuator, or the fixed attenuators built
into your rig. Ideally, you can do this procedure both before and
after putting up an additional antenna that you suspect may cause
By the way, sometimes you can cure interaction by changing the length
of the feedline on the OTHER antenna (not the one suffering the
degradation). The 40-2CD is a classic example of this process. The
antenna itself is not resonant on any HF band except 40 meters.
However, the feedline dangling off the driven element serves as a
which can skew the resonant frequency of the driven element. Since the
driven element is forward of the mast, it can really screw up another
HF antenna if you choose an "unlucky" feedline length on the 40-2CD.
Dave Hachadorian, K6LL
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