Following my success at getting the ATX working the way I wanted, I thought
that I would try and tackle once and for all the problems with the MP-1. I
have never managed to get a very good SWR with this antenna, especially on
the lower frequency bands. Normally, I would just solve the problem by
using the K2's ATU.
In the course of my experiments I found that my note yesterday that the ATX
can be resonated on 30m and 80m using a ground system of four 10ft radials
is probably incorrect. Because the battery of the FT-817 had long since
expired at the time I did that test, I was powering the radio off the power
supply. It would appear that a much better ground was provided by the power
supply than by the MP-1's 4 radials. Disconnecting the power supply and I
could not get a good match. I think that if you want to use the ATX on 30m
or 80m you will need to make up two more tuned counterpoises for those
bands.
Although I am quite pleased with the performance of the ATX with
counterpoises, if you want an all-band portable capability you will have to
carry around a lot of wire which will weigh more and occupy more space than
the antenna itself. It's a pity that I couldn't make the single
non-resonant ground plane idea work, but such is life.
Back to the MP-1, and at this point I switched to the K2, which still had
power in its battery, so I could get isolation from the mains ground.
Disconnecting the radio from the power supply meant I could no longer get a
good match. Moreover, the SWR constantly fluctuated, suggesting that there
was RF on the feeder. Using a coaxial choke solved that, but made the best
achievable match even worse, since for grounding the antenna was now
totally reliant on the 4 short radials.
My conclusion is that the system of 4 10ft radials used by the MP-1
provides a totally inadequate ground for satisfactory operation, especially
on the lower frequency bands. The proof of that is that in A/B tests
between the ATX and a counterpoise cut as I determined yesterday, and the
MP-1 on its standard ground system, the ATX was demonstrably better, by
perhaps getting on for an S-point. (These comparisons were done on 40m and
20m only, due to the lack of any signals on the higher bands.)
A quick test using one of the counterpoises I made yesterday with the MP-1
would suggest that it is possible to get a 1:1 SWR with the MP-1 using a
tuned counterpoise for each band. However, it still appears to perform no
better than the ATX. The MP-1 may outperform the ATX on the WARC bands due
to the fact that the ATX uses a greatly reduced whip length on those
frequencies. However I will leave that comparison for someone else to do as
I have spent enough time playing with antennas for the moment.
As one final test, I did an A/B comparison between the ATX and a Pro-Am 20m
whip. The Pro-Am was mounted on the MP-1's tripod using the 4 10ft radials
plus grounding through the power supply. The Pro-Am was at least an S-point
better than the ATX.
Some final speculation. Although the MP-1 ought theoretically to outperform
the ATX by virtue of its bigger loading coil, you will only get the benefit
of that if you have a very good ground system to go with it. In our
portable application with short wire ground planes or conterpoises, it is
the ground resistance or the counterpoise, not the coil resistance that is
determining performance. Therefore the ATX will probably work as well as
any antenna of similar size used in that situation.
Anyone want to buy a used MP-1? :-)
--
Julian, G4ILO. (RSGB, ARRL)
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