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Re: [TowerTalk] wire antenna question

To: Blair S Balden <>
Subject: Re: [TowerTalk] wire antenna question
From: Terry Conboy <>
Date: Sun, 09 Nov 2008 19:54:39 -0800
List-post: <">>
At 07:07 AM 2008-11-09, Blair S Balden NP2F wrote:
>When I put up dipoles, whether they be "straight" or inverted 
>V's,  I've always started out with the wire a little longer than the 
>formula length of 468/f.  Then I've trimmed for as close to a 1:1 
>match as I can get to my 50-ohm coax, as indicated by my SWR 
>meter.  I can usually get a 1:1 SWR.
>These antennas usually work very well, but I realize that a resonant 
>dipole will not have exactly 50 ohms resistance.  So, I must be 
>cutting these antenna somewhat off resonance.  I also realize that a 
>mismatch at the antenna (SWR of greater than 1) results in loss.
>What I'm not sure about is whether it would be better to get the 
>antenna resonant at the desired frequency and accept the mismatch, 
>or whether I should continue to go for the best SWR (minimizing 
>losses due to the mismatch).
>BTW, my rig does not have an internal antenna tuner, and I am 
>currently without an external tuner for it.  So, I'm running it 
>without a tuner (if this matters).  Also, I have not been using any 
>kind of matching device at the feedpoint, just direct connection 
>from the coax connectors to the wires.
>If someone on here can give me some insight as to whether I should 
>try to go for resonance instead of low SWR, please respond.  I would 
>truly appreciate any help with the question.

In your case, resonance and 1:1 SWR are exactly the same 
thing.  Resonance just means that the antenna impedance is purely 
resistive.  Resonance isn't necessarily the point of highest gain: 
many antennas actually have higher gain when they are much longer 
than the resonant length (e.g. double-extended zepp).

Indeed, a thin-wire straight (nearly) half-wave dipole in free space, 
or at certain heights, will have a resistive component of the feed 
impedance of about 72 ohms.  However, at your mounting height and 
with your surrounding conductors, the resistive part of the antenna 
impedance is 50 ohms.   This is actually pretty common, as you have 
discovered.  (Because of the bending, V-shaped antennas typically 
have a feed impedance under 72 ohms, even in free space.)

Even if your antenna did have a feed impedance of 72 ohms at 
resonance, which would give a 1.44:1 SWR, you can only worsen the SWR 
by changing the length of the wire, which mostly just adds or 
subtracts reactance.

With a 50 ohm line, the ONLY way you can ever get a 1:1 SWR is if the 
load Z is 50 +/- j0 ohms.  (A Smith Chart would never lie.)

In your situation, there really isn't anything else you can do with 
trimming to get higher efficiency.  You already have "nirvana".  Life 
is good.  Be happy.  As they say in Seattle, "keep clam".

73, Terry N6RY


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