My recent experiences with antenna switches may be useful for some of
the readers of this reflector.
I have used commercial and homebrew antenna selection switches for
years. Although I had been warned many years ago that the construction
of commercially available switches was generally poor, I tended to bury
my head in the sand and only observed that it looked like the SWR into a
dummy load increased somewhat as one approached 30 MHz. I assumed that
the higher SWR at higher frequencies could arise from meter errors as
much as from deficiencies in the transmission line system.
Recently, I was gathering benchmark data on a new Steppir Yagi from 20
thru 6M using an MFJ-269. While the SWRs, resistances and reactances
all seemed fairly reasonable from 20-10M, the 6M data had me scratching
my head. The resonant frequency was about one MHz higher than it should
have been. When I checked the 50 MHz SWR into my H/K Cantenna, it was
1.3 but the R was 65 and the X was 17.
To make a long story short, I replaced the Cantenna with a 47 ohm
resistor that had been wired directly into a PL-259 to minimize lead
length. When this load was directly connected to the MFJ-269 at 50 MHz,
I got a 1:1 SWR and the R was 50 and the X was one. However, when I
went through the MFJ switch, an old B&W 501 switch or my own homebrew
switch, I got R's from 35 to 65 and X's from 15-30. (Unlike the MFJ and
B&W, my switch had been made from a heavy duty ceramic wafer and had
large contact surfaces, but it was the worst of the three.)
I then found a Daiwa CS-201 and an old Navy surplus 6-position switch
with N-connectors and checked them. The Daiwa was excellent showing
only an increase in X up to two. Even though I was using an N-to-UHF
adapter, the Navy switch was superb in that the R and X were unchanged
from that obtained with a direct connection to the load.
The two commercial switches and my switch were made with bare wire
connecting the terminals. If they had been made with coax and SO-239
connector hoods or some other kind of construction that maintains the 50
ohm impedance, it is likely their behavior would have been much improved
at 50 MHz. I have no idea how the Daiwa is constructed, but I suspect
that care has been taken to maintain 50 ohm impedance between the terminals.
The Navy switch was unusual in that the common and the switched
terminals consisted of two female N-connectors mounted on a heavy,
rotatable bar and connected by a short piece of coax. The male
N-connectors from the antennas are attached to one of six ports along
the perimeter of the housing. Using the handle attached to the bar, the
bar is lifted, rotated to a new switch position and pushed in so that
female on the bar mates with the male from the antenna.
While this switch worked well on 6M at 200W, it was not a happy camper
when I tried to load a KW into a dummy load on 20M. Like all
N-connectors, it relies on good contact between mating male and female
pins. However, the repeated making and breaking of those contacts in
the switch may be asking too much for routine day-to-day reliability at
I hope this information might be useful for those of you considering the
purchase of antenna switches or having mysterious behavior problems at
VHF with their current switches.
I then did the same I finally isolated the problem in the MFJ-1701
switch. I cleaned up the switch, but there was no improvement. I got
similar results with a B&W 501 and my own homebrew switch.
TowerTalk mailing list