[Elecraft] KPA500 faulting on high VSWR on power rise

[Erik Basilier]

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

-----Original Message-----
From: Rick Bates (WA6NHC) [mailto:wa6nhc at gmail.com] 
Sent: Saturday, February 24, 2018 1:35 PM
To: Erik Basilier <ebasilier at cox.net>
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] KPA500 faulting on high VSWR on power rise

I’ve always presumed that the ratings were based on 1:1 SWR as there is no reasonable way to define it otherwise.   Actual ratings at other loads can be inferred on that basis. 

An end fed dipole is MUCH harder to tame than a dipole (or fan dipole) and inherently challenging to keep the RF away from places like the shack. 

Rick WA6NHC

Smell Czech correction happen

> On Feb 24, 2018, at 11:42 AM, Erik Basilier <ebasilier at cox.net> wrote:
> 
> So, the manufacturer of the protective device should probably not just tell you a wattage rating, but also the acceptable swr level for that power.

> Or abandon the ladder+balun+coax approach and go to an end-fed dipole with transformer that can provide low impedance coax feed on all bands.
> 
> 73,
> Erik