[222mhz] Pre-amp question follow up

[Dr.Gerald Johnson]

Using relay bank or rotary switch up top is uncommon. But it can 
work. There have been some really nice rotary coax switches 
show up occasionally in surplus with frequency capabilities good 
out through 10 GHz. Their trouble is that they are a bit slow to 
change positions.

For four bands, three double throw coax relays at each end 
can be effective. Switch the higher bands the least. Would 
take a little bit of logic, probably can be accomplished with 
diodes working from the band select outputs from the 736 
though as I recall they are active only for transmit. I'll have to 
check the manuals on that. Using relays can band switch the 
antennas and feedline practically as fast as the 736 can switch 
bands. Its not quite practical to listen to two bands 
simultaneously with the one radio. It is handy so long as you can 
stand the noise or neglect the signals that won't open the 
squelch.

I've seen commercial practice (Community antenna TV 
translators) where a few coax cables were used for several 
channels including the local transmitting channels. That's done 
in the Des Moines, Iowa area for the channel 11 and channel 13 
transmitters. There they use a rat race hybrid and some filtration 
to combine the transmitters. I don't remember how the 
multichannel translator mixed the transmitters to the single feed 
line, the antenna was broadbanded enough to work several 
channels.

I've thought of using a similar scheme for ham bands using 
complimentary low pass and high pass filters.

At the bottom, the 6m rig would connect to a 100 MHz low pass 
filter. The 2m rig would connect to a 100 MHz complimentary 
highpass filter, and the outputs of the filters would be in parallel 
and connected to the input of a 180 MHz low pass filter. The 222 
rig would be connected to a complimentary 180 MHz high pass 
filter and those two 180 MHz filters would be connected in 
parallel and to the input of a 300 MHz low pass filter. The 432 rig 
would connect to a 300 MHz high pass filter complimentary to 
the 300 MHz low pass filter and the two filters would be 
connected in parallel and to the feedline. At the antenna the 
opposite collection of filters would separate the bands.

It would be hard to keep the loss through three low pass filters 
on 6m down to the insertion loss of three coax relays. Though 
going to longer yagis can add some gain to make up for the 
loss. Another consideration is that going a couple sizes larger in 
the coax can help make up for the filter or relay loss, and there 
the savings in the extra coax run costs can cover the cost of 
relays or filter parts. Relays will have better band to band 
isolation than ordinary filters unless the filters are specifically 
designed to notch the adjacent bands, which is quite possible 
use the Cauer or elliptical approximations.

Using one run of coax means the wind load on the tower is 
reduced (significant for 1-5/8 and larger coaxes) which can be a 
saving.

Solid conductor coax past the rotor in any size will either pull the 
connector pins apart or break. Solid wire doesn't flex well, it 
breaks rapidly. Stranded coax is needed to go past the rotor.

If anything happens to that one run of coax, you are off the air 
on all bans. With multiple runs (which can be graduated in size 
to use less expensive coax on 6m and the most expensive on 
432) its less likely that all will fail at once.

For lightning protection, build the ham shack next to the service 
entrance panel for the house. With the tower close outside, and 
connect all three with wide (12 to 18") copper strap. Use gas 
tube lightning discharge devices on power and each coax and 
on the rotor cables. They are expensive, but the right ones have 
protected our local repeater with an antenna exposed on top 
a water tower for decades.

In my shack that is at the opposite end of the house from the 
service entrance, I bring all coaxes to a quick disconnect panel 
where I use type C connectors. I have a ground rod on that 
panel, plus a couple at the tower base and one on a guy wire 
plus 8" diameter screw in utility pole anchors for the guy wire 
anchors. When there's lighting about, I disconnect the jumpers 
from the patch panel and make sure there's a couple feet of 
gap. That includes the RF ground that is occasionally needed 
for HF operation. The rigs are grounded through the service 
panel which has a ground rod driven at an angle through the 
lower basement wall, plus the power pole ground that has been 
here 40 or 50 years. With the coaxes disconnected from the 
patch panel, I've heard lighting hit the tower several times with 
no damage to equipment yet. The grounds on the tower and 
guy wires (no insulators) keep the voltage difference between 
the coax feeds and the radios low enough it doesn't jump the 4' 
gap I maintain with the coaxes removed from the patch panel. 
So I shunt the lighting to ground with the tower and patch 
panel grounds, then I insert some series impedance, that of a 
few feet of air gap, to make a two pole lightning filter. Make 
that three poles with the power ground isolated by 30 or 40 feet 
from the nearest tower ground.

Structurally its easiest to put the smallest antennas on top. Don't 
skimp on mast height, there can be a lot of interaction 
between antennas, mostly affecting the higher band antenna 
from the closely spaced low band antenna acting as a ground 
plane just below it or coupling to the harmonic resonances of 
the lower band elements. With 50, 144, and 432 so harmonically 
related that can adversely affect the patterns on the higher 
bands. The 222 and 430 antennas are small enough they can be 
on a smaller sub mast ubolted  to the main mast. I've had a 
pair of K2RIW over the Iowa plains for a couple decades that 
way. My main mast is a 21' length of 1-1/4" schedule 80 water 
pipe with a sleeve of 1-1/2" IMC through the mast top bearing. 
Unfortunately IMC made today hasn't the bending strength of 
the older stuff. It was too hard for electricians to bend so its 
now made weaker. It used to be a good chunk of alloy steel 
that was hard to bend. I'd go a couple sizes larger in pipe 
today.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical 
engineer.
Reproduction by permission only.