One exception is when the dipole is used on multiple bands, then a 4:1
may be required. That was the case for my 370' 80M EDZ (two opposing
5/8 wave, center fed). That is specifically a non-resonant antenna on
any ham band but it performed best with the 4:1 instead of the 1:1. It
was a compromise either way, this was the better choice since I needed
that antenna to work on all bands. If it was only 80M, the 1:1 was the
better choice.
While I am not a fan of MFJ so consider it suspect in this case, I also
believe that since the common mode choke (referred to as a balun in this
thread) should GREATLY exceed the voltage expectations for simple
overhead. The cost differences for higher ratings are not significant.
In my stations, I use the 10KW rated devices from DX Engineering (KPA500
amp) and I've had no issues with them, even at extreme SWR mismatch (QRO
on 160M into the above dipole, 20:1 unmatched with a Heathkit tuner).
My new station will be entirely resonant, so a 1:1 CMC will suffice but
I'll still use the high power devices.
The bottom line here is that the antenna in this thread is not resonant
(because of the feed choice, which is understandable for multiband use),
the voltages presented will be extreme, component overkill (and a more
qualified tuner) should be used. It's not a fault of the amp, but of
the matching network.
It is also KEY that the LEAST amount of coax is used (under 10' to the
final) because the high SWR is present there as well, so the losses will
be extreme and it could even exceed the voltage limits of the coax,
depending on brand, age and type. Been there, done that (darned HOA
limits). This also means that the coax should be tested, from time to
time, until proven worthy.
Rick NHC
On 2/24/2018 9:21 AM, William Shanney wrote:
I have found that a 4:1 balun is seldom the proper choice for a ladder
line interface. I measure the impedance at the interface and then
select the balun type (usually a 1:1). The biggest problem I've had is
the 4:1 transforming the impedance too low for the tuner. This can
result in over stress on the components as the power is increased. If
I don't have an easy to match ladder line interface I add or subtract
line length until I get one.
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