Pete,
Thanks for the description of the setup.
The final missing tidbit of information is a full description of what was
inside the DX-Engineering balun.
The power rating alone is not enough to ascertain if it was the good or
actually the culprit.
Way back in time I used 4:1 voltage baluns because I didn't know any better.
I guess we all did.
I later switched to a current balun (specifically called "Remote Balun") and
supposedly built for this job.
I fried two of those plus several voltage baluns running legal German power
(750w) into various openwire fed antennas.
About 15 years ago, K4TAX recommended I switch to a 1:1 current balun. I
built my own and never had any problem after that.
I had 2 openwire fed antennas (one vertical, the other horizontal) but just
one high power symmetrical matchbox. I tried several asymmetrical
matchboxes including a Palstar AT4K, but had lots of trouble with rf in the
shack, though far less (almost completely gone) after switching to the 1:1
current balun.
About 10 years later I finally learned what was going on with all of this,
reading W8JI's web page.
I was very skeptical of his explanation but I followed his advice - always
use 1:1 with the matchbox, unless you absolutely have to use 4:1, and if you
do use 4:1, it must be a dual-core solution with each transmission line
wound on separate cores. I reluctantly tried this and he was right.
About 5 years later Steve, G3TXQ explained this to me (took 2 tries - I was
slow to grasp it) but now I understand why dual cores are required.
All of this boils down to the differences in how each of these different
types of balun deal with common mode current.
There is an awful lot of mis-information published on the web but thanks to
K4TAX, W8JI, and G3TXQ, I think I finally know what I am doing and why.
Besides RF burns, etc. I was having trouble with my Omni VI+ often tripping
the 961's over-current circuit.
This never happened when using the symmetrical (link coupled) matchbox but
happened often when using an external balun with the asymmetrical tuner.
After switching to a 1:1 current balun and inserting another 1:1 current
choke between the OM6 and the linear, I had no more trouble.
Unfortunately there is no known source of commercial link-coupled
symmetrical tuners anymore.
IMO, the next best thing is a symmetrical T or symmetrical L match.
Worst case, add a good 1:1 Guanella choke at the tx and you should be OK.
Modeling the antenna and knowing its exact impedance at the shack end only
helps if you know exactly what your matchbox is capable of matching. I
don't think anyone can claim that they know that.
At this point I will throw in a plug for Ten-Tec.
With my Model 238B and an external 1:1 current balun, I have been able to
match all sorts of openwire antenna systems, though I often must add the
external capacitor on the back. However I run just 600w these days. If you
run legal power, you will need to replace the high voltage capacitors in the
238.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Peter
Bertini
Sent: Tuesday, December 30, 2014 5:33 PM
To: tentec@contesting.com
Subject: Re: [TenTec] Antenna help
*What were you running Pete, 50 KW? ;-)
Give us some more information on your setup there.*
A very modest setup... Omni VI or Paragon II driving a highly modified
SB-220, through a Dentron tuner.
I hit a very bad combination of feedline lengths and coax lengths between
the antenna, outdoor mounted DX Engineering balun and the shack.
It really is best to have some idea of what is going on regarding the
electrical length of the various feedlines and at the antenna feedpoint.
Whether you are running 50 watts or 1.5 kW, the losses are the same,
although the pyrotechnics are missing at QRP level. The coax losses can be
appreciable as well.
Pete
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