Antenna analyzer and non-resonant antenna
 

I have built an antenna analyzer (AA-908) that reports SWR, complex
impedance, capacitance and inductance. It works fine with coax-fed
resonant antennas.

How can I use it to test a 40M double zepp with 300 ohm twinlead
feedline? I am at this point interested in finding the frequencies in
160M band at which this antenna and particular-length feedline will
propagate.

Needless to say, I see maxed out numbers almost all the time (SWR10
and impedance 600). Should I use a balun? Is there something I can
use (like a 50 ohm resistor in parallel) that will give me numbers
that I can use? What should I be looking for?

Ken KC2JDY

Ken
(to reply via email
remove "zz" from address)

a 10:1 balun would probably help get readings down into a range that your
analyzer can handle, assuming the 600 ohms is the real impedance.

"Ken" wrote in message
...
I have built an antenna analyzer (AA-908) that reports SWR, complex
impedance, capacitance and inductance. It works fine with coax-fed
resonant antennas.

How can I use it to test a 40M double zepp with 300 ohm twinlead
feedline? I am at this point interested in finding the frequencies in
160M band at which this antenna and particular-length feedline will
propagate.

Needless to say, I see maxed out numbers almost all the time (SWR10
and impedance 600). Should I use a balun? Is there something I can
use (like a 50 ohm resistor in parallel) that will give me numbers
that I can use? What should I be looking for?

Ken KC2JDY

Ken
(to reply via email
remove "zz" from address)

Ken wrote:
How can I use it to test a 40M double zepp with 300 ohm twinlead
feedline? I am at this point interested in finding the frequencies in
160M band at which this antenna and particular-length feedline will
propagate.

A 40m double Zepp is 1/2WL on 80m. You didn't say how long the
feedline is. You can get a ballpark idea of the impedance by
downloading the free demo version of EZNEC from www.eznec.com.

Assuming the dipole is 130 ft. long, EZNEC says the antenna
system can be resonated by a feedline length of 96 ft. using
300 ohm line with a VF = 0.9. The impedance seen looking into
the feedline is 0.7 ohms. EZNEC says the antenna has a feedpoint
impedance of 9.3-j1060 on 1.9 MHz with a sky high SWR on the
300 ohm line. This antenna will perform poorly on 160m.

A better solution for 160m would be to short the parallel
feedline together at the tuner and feed the whole thing against
ground, aka Marconi style, with the 40m double Zepp acting as
a top hat.
--
73, Cecil http://www.qsl.net/w5dxp

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"Ken" wrote in message
...
I have built an antenna analyzer (AA-908) that reports SWR, complex
impedance, capacitance and inductance. It works fine with coax-fed
resonant antennas.

How can I use it to test a 40M double zepp with 300 ohm twinlead
feedline? I am at this point interested in finding the frequencies in
160M band at which this antenna and particular-length feedline will
propagate.

Needless to say, I see maxed out numbers almost all the time (SWR10
and impedance 600). Should I use a balun? Is there something I can
use (like a 50 ohm resistor in parallel) that will give me numbers
that I can use? What should I be looking for?

Ken KC2JDY

Ken
(to reply via email
remove "zz" from address)

Ken

I consider dave,s suggestion to build a good transformer to be the best
way to estimate antenna impedance that is beyond the range of your meter.

Is a Smith Chart something you'd use? I cant say which method is best
for your use. But, I'd try estimating the antenna's impedance by
translating back toward the radiator with the assumption that you know. It
is necessary to know the *impedance*, not just the VSWR on the 50 ohm line.
Then, make an attempt to improve the antenna's impedance and see if your
assumptions are appropriate by re-measuring the impedance that results on
the 50 ohm line.

Jerry

On Sat, 12 Mar 2005 08:24:05 -0500, Ken wrote:

I have built an antenna analyzer (AA-908) that reports SWR, complex
impedance, capacitance and inductance. It works fine with coax-fed
resonant antennas.

How can I use it to test a 40M double zepp with 300 ohm twinlead
feedline? I am at this point interested in finding the frequencies in
160M band at which this antenna and particular-length feedline will
propagate.

Needless to say, I see maxed out numbers almost all the time (SWR10
and impedance 600). Should I use a balun? Is there something I can
use (like a 50 ohm resistor in parallel) that will give me numbers
that I can use? What should I be looking for?

Dear Ken,

Here's something else to keep in mind. Your new instrument is designed
and calibrated for use on 50 ohm transmission line. Even if your 300
ohm twinlead were matched properly to an antenna, and there are no
standing waves on the line, your instrument will still show an SWR of
6:1


Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html