[AMRadio] antenna tuners


Gary Schafer garyschafer at comcast.net
Thu Apr 20 11:08:25 EDT 2006


Hi Alan,

No need to go away! You will get lots of good info here and asking /
discussing is how to understand.

As to resonant antennas, it makes no difference in how well they radiate.
Resonance of the antenna is not required.

SWR is not all that bad either as long as the transmitter sees a match,
especially the solid state rigs. 
When open wire feed line is used there is usually very high SWR on the feed
line. If you connect 450 ohm line to a half wave dipole which is normally in
the 50 to 70 ohm range you have high swr on the feed line. And no, high swr
on a feed line will not cause it to radiate.
What causes feed line radiation is an unbalance between the two wires in the
feed line.

High SWR on coax line will cause a little more loss in the feed line due to
the higher currents involved across the lower impedance of the coax. That is
why when using open wire line that is 400 to 600 ohms, there is much less
loss. The same power across a higher impedance means less current and less
current going through the feed line wire means less power loss.

An antenna tuner just matches the impedance seen at the transmitter end of
the feed line to the 50 ohm transmitter output. If there is high swr on the
feed line, any power reflected from the antenna is not wasted it is just
re-reflected back to the antenna and eventually gets radiated.

At HF even fairly high swr on coax lines does not cause excessive loss.

73
Gary K4FMX


> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Alan Beck
> Sent: Thursday, April 20, 2006 5:45 AM
> To: AMRadio at mailman.qth.net
> Subject: [AMRadio] antenna tuners
> 
> I don't believe in them.
> 
> If you can't resonate your antenna, then what are you doing?
> 
> If you use a tuner, you are creating a voltage divider effect that
> creates a reactive load in your shack, to ground that makes your antenna
> "feed line + radiating elements".
> 
> Over the years, the importance of having at least a 2:1 match with at
> least 1.5:1 some where in the the antenna design.
> 
> Proper antenna design for your favorite frequencies is the best choice.
> 
> I currently use a multi-element dipole to cover 80,40,20,15 and 10 meters.
> 
> I just use a VSWR bridge and back off the power when the SWR rises close
> to 2:1.
> 
> I am not saying my answer is the best, I am only stating my opinion.
> 
> Just like microphones in the sound work I help out with at church. If
> they are not singing into the mic, I cannot "fix it" without creating a
> gain structure that is sucking up to feed back"
> 
> You need to fix your problems at the source.
> 
> I our cases, it is resonance of your antenna, by some means.
> 
> Even my hamstick on my jeep does great due to attention to resonance.
> 
> 73
> Alan
> VY2WU
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