[TowerTalk] (long) Summary:Commercial antennas
Tom Frenaye
frenaye@pcnet.com
Sat, 31 Jan 98 17:40:46 EST
Thanks to the following people for their input!
AA1K K1VR KA1CLX KF4HK KH7R KL7HF N6IG NJ2L W7NI WA6MXI ZL1ANJ
My original message:
I've been talking with someone about placing a commercial paging antenna
(931 MHz) on one of my towers. I'm looking for specific suggestions for
things to watch out for regarding their RFI potential - me to them and vice
versa. I'd like to minimize any problems before we get started - anyone
with some direct experience?
Most of my operating is 160-10m but I do have 50, 144(packet) and
430(packet). One tower being considered has 40m/10m beams plus one 430
antenna and my 160m 4-square around it. The other has 20m beams plus my 80m
4-square. Only one of my Beverages comes close to either tower (about 30'
away).
- K1KI
Responses:
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Tom, I wouldn't worry too much about interference from this,as it is so far
removed, they usually have really good filtering anyways. XXXX down
near you has commercial stuff at his site,you may want to talk to him, but
I don't think he's had any problems.
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Go for it, but I advise:
1) Have them install at least a single stage isolator and a bandpass
cavity on the TX. Most 931 TXs are clean, but this is good insurance.
We require such devices on all TXs at our sites.
2) They will likely require a link RX of some sort. Most pagers used to
link on 72 MHz, but at least out west they all seem to distribute via Ku
band satellite. So they may want to put up a small dish at the site to
receive the signals. Be sure they use good coax on the baseband feed
from the dish to the transmitter.
3) If I were you, I would be more concerned about personnel safety. Most
paging transmitters will be on the air 24 hours per day, and most seem to
run at least 250 watts output (some 500 watts) into 9 or 12 dB gain
antennas. That will exceed safety limits as you climb past it to do
tower maintenance. Make sure you have some sort of clause in your lease
to allow you to turn their TX off when you want to climb your tower.
4) Obey all "common sense" rules (even though common sense isn't that
common any more). Only let then use solid shield transmission lines
(heliax on the tower, superflex jumpers between the TX, the isolator and
the cavities), ensure they have lightning arrestors on the antenna
feedline, ground kits on the coax at the antenna, where the antenna
feedline bends off the tower, and where the feedline enters the building,
etc.
5) Since their TX will eventually be on the air 24 hours per day in a
busy simulcast system, make sure you take into account the heat load of
their TX on your HVAC system (along with the associated increased
electric bills) as well as the electric charges for their TX. Of course,
if they have their own meter, so much the better.
6) If you want to court more than one paging company, you may want to
install your own antenna system on the tower, and invest in a TX
combining system. Sell them ports on the combiner, and jam several
pagers into one antenna. Then, since you own the combiner, you can
depreciate the investment...
If you end up renting to them, I will send you a bill for my consulting
time! ;=>
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You may want to have someone make discreet inquiries of zoning.
Here, zoning changes if amateur service changes to any commercial
aspect.
Then setbacks, fees, etc. all change.
----------------------------------------------------------------------------
Hi Tom. I'd recommend against it.
You're really asking for headaches. But - if you decide
to go ahead, insist that ferrite isolators are put on each
line, including your amateur gear. What isolators do is
to allow rf to travel only in one direction thru the device,
and the first port is the radio, second port is the antenna
and the third port is a dummy load. Rf from the tx will
go to the antenna, but rf coming back from the antenn
will be directed to the dummy load..
In addition, resonant cavities should be installed in each
vhf and upward line.
Next - all excess hardware should be removed from the
tower. Any metal that is in contact with the tower and not
firmly attached is a source of intermod.
Insist on a clause that holds you harmless in case of interference.
I have an intermod program that I would be glad to send you
for looking at the obvious combinations. But, there are always
the unexpected.
Again, I would think long and hard about what you're getting into.
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One small consideration is that you will probably have to guarantee
7days x 24 hours on-time for the commercial setup.
This may make it difficult to close it down when you wish to
climb the tower to service the amateur antennas. I would prefer
to keep a respectful distance from a cellular antenna.
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I was involved in finding sites for a paging company operating on 931
and can alert you to a few things. First, make sure in your written
agreement with them that their operation is on a non-interference basis
and that operations must cease immediately upon notification until the
problem is resolved. Second, make sure that in the event separate power
is not provided that you have the right to pass on the cost of
electricity. Usually, each 931 mhz. transmitter will run an additional
$80 to $100 a month depending on the duty cycle. Third, due to the
high power level they transmit, you don't want to have them put their
antenna on the tower where you have to climb right by it. It would be
best to mount it on the very top of the tower. By the way, most 931
paging operators use remote monitoring via phone lines so any
interuption to power, antenna systems, phone lines etc. will bring a
technician responding within hours. As far as interference is
concerned, its going to be trial and error. I have seen cases where
there were no problems and other cases where there were.
p.s. - You also have to make it clear to them that any costs to resolve
interference problems are to be their responsibility, not yours.
I actually had a case where a site owner had an RFI problem caused
by the paging company and had to pay to resolve it himself because
it wasn't in the contract.
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RE: Adding 931 MHz to your amateur tower is a great financial idea... but
have you looked at what it might do to your use of the tower from the
cities permitting viewpoint. Be sure your not going to require building
permits, conditional use permits, etc. If you are required to have these
you could be opening the 'can of worms". I have commercial equipment at my
home qth but on several towers. The commercial towers had to go through
the permitting process. I have spent a good deal of money to do so... more
than only one transmitter could off-set for a long time. In my facility I
have 22 different transmitters and now it pays the mortgage! I don't have
any amateur radio stuff on the commercial tower an visa-versa. In many
cases amateurs get a break with their towers in residential districts but a
commercial venture is not allowed. Don't get me wrong. The idea is one
that many hams have done but be sure you don't get into hot water. You
will need to become VERY familiar with FCC document OET-65 and you should
know the dangers of 931 MHz paging transmitters. For one thing you can't
just shut them off to work on your other amateur antennas at will... and
they almost never are off for more than a few seconds and then back on
again. Most run fairly high power ... like 300 to 500 watts output into a
gain antenna so your looking at 1 to 2 kW ERP! Hope this gets your mind
going in the right direction. Good luck and be careful. I have 11
commercial sites here in northern California. Each site has it own special
circumstances. Leases are quite a bit for the inexperienced. Mine is give
or take eighteen pages long. There's a lot you need to have in there to
protect yourself and your family. Again good luck.
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I DO have direct experience with this very thing. About 6 months ago, a
company approached me and asked if I would consider letting them put a 900
MHz receiving antenna (not transmitting) on one of my towers. We negotiated
for $250 per month and they installed it. They also installed two phone
lines and a receiver in a box that is about the size of a two foot cube. It
plugs into the wall but does not take much juice. I pay the electricity.
It has backup batteries in case the power goes out here. They have never
failed to pay their bill and I enjoy the free money.
Unfortunately, the stuff you REALLY want to know about, I have NO experience
on. I have not had even one QSO since it has been installed. I thought
about it and I could not imagine how I could interfere with it nor how it
could interfere with me. I have not fired up and tried to blow it away with
the amplifier so I can't say for sure what would happen . . .
I do suggest that you keep it a closely guarded secret, however. If a
couple of people around here who are anti-antenna knew I had a commercial
antenna on my tower, they would raise all sorts of hell and claim this is
residential and not zoned for commercial, etc. I told the antenna
installing company they would be much better off if they did NOT apply for a
permit from the county to install it. Nobody could tell if from just
another ham antenna and I DAMN SURE wasn't going to tell.
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Hi Tom...I just moved and half a mile from my QTH is a 249-ft commercial
tower which has 800 mhz trunking systems on it. I suspect there also is a
paging system on it since on the 2m HT (TH22AT) that I'm temporarily using
for packet access I have noticed paging signals coming through strong on
several packet frequencies (144.91 and .93 if I recall, and some others,
but fortunately not on my own).
My 2m ant. is a 3-el yagi broadside to the commercial tower.
This may be sheer overload of the HT that a stub would notch out once I
determine the source frequency.
I did a similar survey last year when a cell company was considering my old
place for a site, and had reports of many hams co-existing without any
problems. But cell-phone operations are fairly low power; I think paging
systems run quite a bit more so they can reach those no-antenna pages, hi.
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>From my direct, personal experience, you have absolutely nothing to
worry about from an RFI standpoint. These pagers typically run 100 to
600 W, plus 10-dBi-gain omni antennas, so their EIRP can be 1 to 6 kW;
even so, you won't even know the stuff is there unless it breaks and
puts out power way off frequency from where it should. However, I would
insist that the paging company install an isolator on the transmitter
output.
Be aware of the legal potential here. Allowing a commercial user to rent
space on one of your towers may change your property classification from
residential to something else, which has been known to give towns an
invitation to hike property taxes drastically.
I don't know if this would apply in West Suffield, but a good part of
that probably depends on whether the paging company has to apply for
some kind of a permit to do this that would disclose that their
equipment is on your property.
Don't worry about RFI, unless you want to operate on 903 MHz some day
(and you can expect such an animal to wipe out a 900-MHz cordless phone
or similar appliance). Do worry about the tax potential. Also keep in
mind that you will need to work on the tower while the paging
transmitter is running, and I certainly would not feel comfortable doing
this even if it's a relatively QRP transmitter (under 300 W). Perhaps
you could work out something with the paging company to take their
transmitter down for a few hours at a time. This is not going to be
something they want to hear, so get it in writing, in advance, and make
sure they make the necessary provisions for you to ensure that the power
is *off* from outside their box.
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E-mail: frenaye@pcnet.com
Tom Frenaye, K1KI, P O Box 386, West Suffield CT 06093 Phone: 860-668-5444
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