[Elecraft] KPA500 faulting on high VSWR on power rise

[Rick WA6NHC]

I have an R7 (end fed vertical 40-10M).  It and a 80M dipole are my 
current antennas, until I plant a tower later this year (55' US Tower, 
motor drive, tilt over).  I have to admit that I'm not a fan of vertical 
antennas.

Since I moved here last year and haven't gotten the tower up yet, the R7 
is 'adequate' for a temporary antenna, but dipoles are more effective.  
It also provides decent diversity reception with a horizontal dipole.  
(The new antenna, when it gets up, is a SteppIR DB-42 at 60', resonant 
80-6M then I'll add in a Beverage for RX only since I have the space now 
or a K9AY.  I have a 160M inverted L up too (bend at about 90'), using 
the underground water pipe as the counterpoise, buried down 4-5', 1:1 
CMC at the base, it was quick and simple.  I'm adding a tuning network 
to that so that will be a 80/160M resonant vertical, giving me another 
option on 80M.)  The beautiful part is that with only 2 resonant 
antennas, I can cover 160-6M once I'm done, perfectly matching the new 
KPA1500.  Simple ROCKS!

The R7 (like the R5, 8, 9) has a matching network (torroids mostly) in 
the black box and trap and capacitive elements on the main element and 
short radials at the base to get it to load, but that doesn't meant that 
it's efficient; it means it presents a load that is acceptable (so does 
a dummy load).  The EDZ at my last QTH beat  the R7 in almost all 
scenarios on any band, unless the other station was off the ends of the 
dipole, then it was even money; which left the R7 being used for diversity.

So it works, but I wouldn't rate it highly unless one is limited by 
space, HOA or similar.  It reacts badly when near (w/in 10' of) any 
metal and the base must be at least 10' AGL.  Adding more choking is 
very wise.  I would expect similar results from a non-commercial 
antenna, unless used only on one band (and it's still a challenge to tame).

Dipoles are much simpler, even when used for multiband operations.  Cut 
the antenna to the desired (available) length, add the feedline, 
measure, adjust for local conditions (add/cut feedline for open wire or 
antenna for single band) then see how it plays.  Add common mode choke 
transformation (1:1 vs 4:1) to keep the currents outside the shack, feed 
with no more than 10' of coax to the final; have fun.

Reality beats textbook every time since the textbook doesn't know about 
the septic field, tree proximity or the dog kennel fence, let alone 
actual height above ground which isn't always the top layer of dirt.  Go 
play, have fun, it's part of the long tradition of ham radio.

73,
Rick NHC


On 2/24/2018 3:45 PM, Erik Basilier wrote:
> A lot of users seem pleased with end-fed wire antennas of recent commercial models. Such antennas should not behave much differently compared to end-fed verticals such as the R5 that I have had for many years and which creates no problems even at high power. Like that antenna, the wire models now becoming popular use a high ratio wide-band impedance transformer. The R5 also includes tiny radials and a common mode choke at the feedpoint. The R5 behaves well even at high power with its feedline length of maybe 50ft, and there is not enough RF in the shack to affect the operation of radios or other equipment. However, for good measure I added a second common mode choke in the form of several ferrite snap-on's about 10 ft from the feedpoint and then found a noticeable reduction in shack RF (measured in the shack using an MFJ RF current sensor). I believe there are lots of satisfied users of the Cushcraft endfed verticals. As to the wire end-fed's I am constructing an imitation of a popular commercial version, and will find out for myself if there are any difficulties. I will add a common mode choke placed either at the feed point (like the R5) and a separate counterpoise, or I might try placing the choke a distance away from the feedpoint to let a portion of the feedline act as counterpoise. Again, a second common mode choke further down the feedline will likely be added. Based on the positive reports by so many users of the commercial versions, who apparently don't add any common-mode chokes, I am fairly confident of success.
>
> 73,
> Erik K7TV
>
>