Let's say I have a free-standing vertical HF antenna transmitting about
10wa, and I'd like to run wires to something at the tip. Say an
anemometer, or a temperature sensor. What would be the best way to run
the wire up the antenna? If the HF vertical is a pipe, is inside the
best bet? If it's shielded wire next to a solid antenna, will that
affect the radiation pattern of the antenna as a parasitic element? How
much energy at the tip of the antenna will couple into the wire? If the
wire is carrying low-speed digital data (eg for a 1-wire temperature
sensor), how much isolation would it need (choke, AC couple, opto-isolate,
etc) to avoid getting the HF into the digital signal?

Thanks for any insight!

--
Ben Jackson

http://www.ben.com/

Ben Jackson wrote:
Let's say I have a free-standing vertical HF antenna transmitting about
10wa, and I'd like to run wires to something at the tip. Say an
anemometer, or a temperature sensor. What would be the best way to run
the wire up the antenna? If the HF vertical is a pipe, is inside the
best bet? If it's shielded wire next to a solid antenna, will that
affect the radiation pattern of the antenna as a parasitic element? How
much energy at the tip of the antenna will couple into the wire? If the
wire is carrying low-speed digital data (eg for a 1-wire temperature
sensor), how much isolation would it need (choke, AC couple, opto-isolate,
etc) to avoid getting the HF into the digital signal?

Thanks for any insight!

Hello Ben,

If it were me I would find a way to attach it to my roof vents or
somewhere else for sure. I'm just real skittish about damaging or false
readings from my wxr station because of my rf. Gud luck to you.

Butch KF5DE

Hi Ben

Keeping on mind that the data rate going to/from the sensor is very low
compared to the RF I *think* they will co-operate. Some thoughts though;

- Opto systems have the problem that you need a power supply for the
remote end. Using optical fibre is even a more elegant idea than wires
but has a high-ish setup cost. You could of course use a IR
LED/optotransistor config but as stated the power supply becomes an issue.

- You could argue that infrequent TX RF on the antenna even if it
affects the sensor (data) can be ignored . This of course depends on
what you are using the weather data for.

- If the weather data is a DC/AF level only you can allow RF onto the
connection wire. Dont bother to try isolating it from the antenna at
all. Dont even think of using shielded/coax. This way the effect on the
antenna RF use will be small(er). You could get away with a choke/LPF or
two at the antenna base, perhaps even with the use of coaxial stubs.
(I'd start with a 1/4 wave open coaxial stub between each wire and
antenna metalwork ground (ie where the antenna feed coax joins, then a
other two further down)) Running the wire on the inside would of course
be best for aesthetic and weather damage considerations.

- I'd say the effect of a base decoupled wire on the antenna would be
neglible next to that of the aneometer or any box at the antenna tip.
You would be capacitively loading the antenna at the "worst" place.
(assuming a current node like at the tip of a 1/4 wave) ie it would need
a retune.

Cheers Bob VK2YQA

Ben Jackson wrote:
Let's say I have a free-standing vertical HF antenna transmitting about
10wa, and I'd like to run wires to something at the tip.

Is this a multiband antenna? If so, I'd worry about the wire coupling
different parts of the antenna to each other differently and requiring
retuning at best, making it impossible to get a match at worst.

If it's a single band vertical, or at least just a single pipe without
traps then inside is the best bet. What you probably want to do is put
an RF choke both at the top and bottom.

Actually, I would suggest NOT putting the sensor at the top of a
vertical, but rather put it a few feet from the top. The tip of the
antenna is a high voltage point, and the top choke will be much more
effective if it's at a lower voltage point where some current is
flowing.

Run the wires up inside, use twisted pair, put a hole in the side of
the antenna at a few feet from the top and attach an insulating rod
sticking out sideways from that point. Put a good choke on the wires
where they exit the antenna element both at the hole near the top and
at the bottom of the element.

If it's a trapped vertical or something you'll have to worry about the
extra coupling between antenna segments due to the wire.

If you're only running 10 watts (I think you said), I doubt you'll have
to worry too much about arcing and so forth in the trapped situation,
but it will probably detune the antenna quite a bit even if you run it
inside.

73,
Dan
N3OX

Oh and I like the fiber optic idea too... how about a solar + batteries
power supply for the anemometer...

hehe... probably too complicated ....

I do have some feet of optical fiber that was pulled from some old
piece of lab junk and I'm saving it for something like that but I
haven't really thought of anything yet.

73,
Dan

Ben Jackson wrote:
"Let`s say I have a free-standing vertical HF antenna about 10 wa, and
I`d like to run wires to something at the tip."

No problem.

All the stations I`ve worked in, medium wave and shortwave, had lighted
towers. Some also used the towers to support other antennas and devices.
Medium wave towers have sampling loops bolted to them for remote current
and phase indications.

It`s dead simple. Conduit and coax are used to protect wires to devices
and other antennas. Conduit and coax are firmly connected, top and
bottom, at least, to the tower to shift most of the lightning to the
lower-inductance tower.

At the bottom across the base insulator, coax is coiled to create a
reactance at least 10x the feedpoint impedance of the tower. Similarly,
each of the other wires running up the tower for illumination or other
purposes is broken at the base for a tower-lighting choke (or Austin
transformer) to isolate the RF and lightning. It works like a charm.

Best regards, Richard Harrison, KB5WZI

On 2006-05-19, wrote:

Actually, I would suggest NOT putting the sensor at the top of a
vertical, but rather put it a few feet from the top.

Actually, for temperature, that might be the best idea yet. I could
run the wires entirely inside, stop somewhere away from the top, and
even disconnect the sensor with a PTT-triggered relay. The application
is a beacon, so the transmit will be very intermittant anyway. I'd
like to keep the entire antenna enclosed in PVC, with nothing hanging
on the side, which is what got me thinking about INside.

Thanks for all the replies.

--
Ben Jackson

http://www.ben.com/