----- Original Message -----
From: "Rick Bullon" <kc5ajx@hotmail.com>
To: <towertalk@contesting.com>
Sent: Sunday, November 07, 2004 8:36 PM
Subject: [TowerTalk] Wireless internet antenna on ham tower
> Last week I learned that in the next 2 to 3 weeks we will have high speed
> wireless internet service available ( hooray). I stopped by the local
> computer store that was offering the service. The owner took me into his
> office and did a line of site survey on his computer, based on my location
> and the tower that is going into service later this month. Well according
to
> the tropo program his is running there is a large hill between me and the
> tower site that will block the wireless signal. (which I find hard to
> believe because the tower is the same one that the local 2 meter repeater
> is on and I can hit that full quieting with a handheld and 2 watts)
What frequency is the WISP operating at? Propagation at 2.8 GHz is quite
different from 2m. Diffraction is probably the biggest difference. The
wavelength on your 2m HT is 20 times longer.
You also have a very different SNR situation with the 2.8GHz. You only need
about 10 dB SNR in a 15 kHz bandwidth for full quieting on the 2m repeater.
Presumably you're expecting broadband type data rates, so the microwave link
will need a bandwidth of several MHz, which will bring up the noise floor
by, say, 20 dB (1.5 MHz vs 15 kHz). Your flat panel antenna is probably
something like 16-20 dBi gain and there's another one like it at the other
end presumably. However, those gains are for isotropes, so you also have to
figure in the effect of the smaller aperture for an isotrope. 20 times the
frequency works out to 26 dB more path loss. Other factors too...
> Anyway he said that he could not service me as there was no line of site
for
> the signal. I mentioned that I had a 70' tower ( Rohn 25) and he ran the
> program again with the receiver antenna at 50' on my tower. Now we had a
> line of site.
> Now the questions
>
> 1 I was told the internet antenna was about 2' square and the signal was
2.8
> ghz. My tower has a TH-6 @40' another TH6@70' and a F-12 EF240X@about 80'
I
> don't think this little antenna has small enough wind load not to over
load
> the tower right?
>
> 2 Since I am going to only be active on HF then I should have to worry
about
> interference from or to the internet antenna right?
Yes you do have to worry. How much HF power can the front end of the
internet antenna/receiver take? Sure, the aperture is small, and the HF is
out of band, but I'll bet that the designer of the microwave system never
really designed for being in the near field of a gain antenna radiating a
kilowatt.
Likewise, there may well be a switching power supply in the beast (to run
the transmitter), and it might radiate a fair amount of RF. In the usual
installation where it's 20 feet up in the air, and nobody has a sensitive
receiver near by, this might be fine. You're putting it close to the feed
point of an antenna connected to a sensitive receiver.
>
> 3 Now this is where I am concerned I plan to shunt feed the tower for 160.
> If I decide to run QRO on 160 do I need to worry about frying the internet
> antenna???
Yes.
>
> 4 in the future I need to get an antenna up for 80 meters I have a full
size
> dipole I can use for now. Later I am thinking about 4 slopers any problems
> here??
>
> The guy at the computer store said that I may in a good position for an
> internet node here at my QTH. Do I really want to consider this what are
> the pro and cons of this? I seem to remember a thread on here about
someone
> being approached by a cell phone carrier to use his tower for a cell link
> but I think this is a little different as the antenna is smaller. I
already
> know about getting them to take responsibly for inference problems as they
> are unlicensed and I am the licensed party anything else?
>
> I also know to make sure they have proper insurance in place before I let
> them on my tower anything I am forgetting anything?
I would definitely ask them for some experimenting time before committing to
anything (on either side). Can they put up their gear as a jury rig lashed
temporarily to your tower for a electromagnetic compatibility testing? I
assume the equipment is FCC registered (either Part 15 or type accepted), so
there are EMC test files you could look at to assess things like HF radiated
emissions.
Jim, W6RMK
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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