[Elecraft] half wave vertical

[Thomas Beaudry]

Although the impedance seen by the feed line will change depending on 
where you attach to the antenna, the radiation resistance will not.  A 
half wave, single conductor, resonant antenna has a radiation 
resistance in the neighborhood of 50 ohms when near the ground.

So the efficiency for your example would actually be 50/(50+100)=33%.  
That is of course assuming a totally lossless matching circuit.  If you 
can determine the loss of your matching circuit, add that to the 
denominator for a more accurate figure.

Increasing the length to 5/8 wavelength will increase the efficiency a 
little bit more.

The single best thing you can do to improve the performance of a ground 
mounted vertical antenna that is 1/4 wavelength or longer is to reduce 
the ground resistance.

Even so, the 9% improvement by going with a 1/2 wavelength instead of 
1/4 is worthwhile if you have the means to do it.  If you are 
interested in DX, the improvement is even more significant as the 1/2 
wavelength antenna will put more of the signal at or below a 20 degree 
take off angle than will the 1/4 wavelength antenna.

> Since ground resistance is in series with the radiation resistance of 
> the antenna a higer Z antenna makes the ground resistance loss less 
> significant.  His example: 1/2 wave vertical Z (end fed)=3300ohm. With 
> a ground rod and a few radials ground resistance about 100 ohm.  
> Efficiency about (3300/(3300+100) =97%.  For a 1/4 wave vertical Z 
> (end fed)=32 ohm and with the same ground, efficiency is 32/(32+100) = 
> 24%!!

-- 
Thomas M. Beaudry
k8la / ys1ztm
K2 # 3422