[Elecraft] Antenna Quandary

[Jim Brown]

On 5/11/2022 6:51 PM, Richard wrote:
> Jim —
> 
> I just finished building a half-wave 40-meter dipole and have it nicely 
> tuned for the middle of the SSB segment. Out of necessity, the tuning 
> process was carried out with the antenna horizontal.

Feedpoint Z of horizontal antennas is slightly affected by height and 
surrounding objects, so ideally should be tune at the rigged height, 
then lowered and re-adjusted if necessary.
> 
> *QUESTION 1:* Should I ever hang this antenna as an inverted V, would 
> that result in a significant change to the location of the sweet spot?

Slightly.
> 
> When we finally got the leg lengths to the point where the SSB sweet 
> spot was damn near perfect, we had folded back 30 inches of excess leg 
> wire on each leg. The excess was not twisted around the legs but instead 
> laid parallel to the leg wires and Gorilla taped in place in several places.

Remember that SWR is NOT a measure of antenna performance, it only tells 
us approximately where it is resonant. That's because the feedpoint 
impedance varies a bit with height, thanks to the reflection from the 
ground coupling back to the antenna.

Folding it back as you have done is perfectly good technique -- the 
antenna ends at the fold (that is, where it goes through the insulator. 
We routinely do that when rigging new wires.
> 
> *QUESTION B:* I’d like to cut off 12 of the 30 inches of excess wire, 
> leaving 18 inches in case I or some future owner wants to retune down to 
> the CW segment. Will shortening the excess wire from 30 to 18 inches 
> influence the sweet spot?

SWR matters two ways. First, the match at the transmitter must be good 
enough that the transmitter can put full power into it. Additional loss 
due to SWR doesn't matter much until the SWR gets pretty high unless 
you're using small diameter coax or it's a long length of coax. As long 
as you have an antenna tuner (really an adjustable network to match the 
transmission line to the rig), any SWR below about 2.5:1 is plenty good 
enough unless the line is long enough, or small enough diameter, to have 
a lot of loss. That's why smart hams use big coax like RG8 or RG11 to 
feed dipoles. Low dipoles are closer to 50 ohms, high dipoles are closer 
to 75 ohms. Low would be much less than a quarter wave, high would be 
much more than a quarter wave. That feedpoint Z also depends on ground 
conductivity.

SO -- since the full width of 40M is a small as a percentage of the 
frequency, so a half-wave dipole cut for 7150 will be just fine to cover 
from 7,000 - 7,300 kHz. Ditto for all the bands above 40M until you hit 
10M, but even there, unless you work FM, which is at the top of the 
band, a dipole tuned to about 28.45 MHz will be "good enough" for where 
all the action is.

BTW -- as study all this stuff and learn to use the tools, there is 
antenna modeling software that will let you learn by modeling.

Here's a study I did several years ago on the effect of antenna height 
both as text and slides.
http://k9yc.com/AntennaPlanning.pdf
http://k9yc.com/VertOrHorizontal-Slides.pdf

73, Jim K9YC

> Looking forward to your input.
> 
> Cheers.
> 
> Richard
> 
>> That said, what's your question? OF COURSE there are LOTS of folks 
>> here who know a LOT about half wave dipoles. I have six of them in the 
>> air for different bands and directions.
>>
>> 73, Jim K9YC