[Elecraft] Folded Dipole Antennas

Mon Dec 8 12:28:00 2003

The 0.5 dB tuner loss was determined by the Frank Witt AH1I indirect method
described in the Set/Oct issue of QEX. I repeated my tests last evening on
another project and came up with 0.55 dB this time on 14.095 MHz. Witt
claims his method to be independent of frequency over the 160-10 meter
range.

My baluns are all 1:1 current baluns using ferrite beads over coax.

I use non-resonant (20-meter EDZ) antennas, and the EZNEC models predict
very workable driving point impedances. The 35 ft of ladderline plus the
balun and then 4.5 ft of coax to the tuners shows a maximum VSWR of 10:1 or
less.

You are correct in that certain combinations of antenna length, frequency
and feedline length can lead to some interesting and troublesome line input
impedance to try to match. The goal is to design the entire system to
minimize this issue as well as the loss in the feedlines. The approach is
careful design and selection of antenna and feedline lengths.

In general, there is little advantage and considerable disadvantage in
trying to feed a resonant antenna with ladderline. Your example of using a
one wavelength antenna and having to deal with very high SWR demonstrates
why I never use resonant antenna lengths for multi-band operation. What
works well on one band may be an impossible condition on the second
harmonic, commonly the case with resonant 80 meter halfwave antennas which
become fullwave antennas on 40 with the problems you describe.

73, George W5YR

---- Original Message ----- 
From: <[email protected]>
To: <[email protected]>; <[email protected]>; <[email protected]>;
<[email protected]>; <[email protected]>
Sent: Monday, December 08, 2003 10:10 AM
Subject: RE: [Elecraft] Folded Dipole Antennas

What impedance is the 0.5 dB measured into? A full wavelength antenna will
present an impedance of a few thousand ohms. This is outside the desirable
range for a 1:4 balun. Now on some bands the VSWR can be nearly 100:1. This
is where the loss of even open line can climb to a dB or more.

This would make an interesting experiment to measure the actual losses on a
typical center fed zepp.

   Dave WX7G

-----Original Message-----
From: George, W5YR [mailto:[email protected]]
Sent: Friday, December 05, 2003 4:12 PM
To: drcuthbert; [email protected]; [email protected];
[email protected]; [email protected]
Subject: Re: [Elecraft] Folded Dipole Antennas

Interesting . . . my tuner measures 0.5 dB loss on 20 meters and the 36 feet
of ladderline to the 20-meter EDZ surely doesn't add more than a fraction of
a dB more loss. Seems that a system like mine that displays "a few dB" of
loss would need some work.

George W5YR

----- Original Message ----- 
From: <[email protected]>
To: <[email protected]>; <[email protected]>; <[email protected]>;
<[email protected]>
Sent: Friday, December 05, 2003 4:12 PM
Subject: RE: [Elecraft] Folded Dipole Antennas

You won't measure the 800 ohm resistor as it is fed thru a 1:16 balun. The
cost of the resistive termination is 5 dB or so. About like turning a 100
watt rig down to 30 watts. Not that bad unless one is trying to win
contests. Now contrast it to a comparable ladder line fed zepp with line and
tuner losses of possibly a few dB. Now there isn't that much difference.