[TowerTalk] Broadbanding 80 meter dipole

[Jim Brown]

On 8/19/2017 7:28 PM, Dan Maguire via TowerTalk wrote:
> Grant KZ1W wrote:
>>>> The ARRL Antenna Handbook has a number ways to broadband a dipole, several probably difficult as noted.  One not in the current handbook is the Witt/Leeson tuned coaxial resonator feed method, see ...
> And Jim K9YC followed up with:
>>>> I included the technique in a talk I did some years ago on using SimSmith to design antenna matching networks.
> This broadbanding technique came up a few years ago here on TowerTalk and was used as the basis for an AutoEZ modeling study.  See the fourth bullet here
>
> http://ac6la.com/aecollection5.html
>
> and be sure to note the caveat raised by Steve Hunt, G3TXQ, at the end showing why this technique won't work in some situations.

G3TXQ correctly observes is that the technique isn't useful for 
electrically low dipoles, because their low height causes their 
feedpoint Z to be closer to 50 ohms.

There's another effect -- feedpoint Z includes the mutual impedance of 
the ground reflection. This effect is what causes the feedpoint Z of a 
horizontal dipole to "oscillate" as its electrical height is varied. 
AND, the strength of the ground reflection is affected by soil 
characteristics.

Example: For our county expeditions for CQP (California QSO Party), we 
rig dipoles for 80 and 40 in scrub trees. Our site for several years has 
lousy soil. When I modeled our 40M dipole at what I expected would be 
its mounting height (about 35 ft) over good ground, feedpoint Z was 
close to 50 ohms, but when I modeled it over poor ground, it was 75 
ohms. When we rigged it, field measurements confirmed that it was a good 
match to the RG11 we had brought to feed it.  This mattered, because the 
feedline length was about 250 ft (this dipole was for the SSB station, 
which needed separation from the CW station's dipole).

Thanks for all of the great work you share with the ham community, Dan.

73, Jim K9YC