[MRIC] A little help

[Me]

Bob,

I assume your email was intended for everyone to get a wide variety of suggestions, so here's mine, based on experience of dozens of commercial and public safety installations:

I will just comment on the VHF/UHF antennas since I am still working on my HF antenna systems, except to say that I am considering two simple wire dipoles cut for 40 and 80 meters (so we can use them without antenna tuners if we have to), center-fed with coax (because I am paranoid of accidental stray HF in the building affecting computers and 911 dispatch consoles).

Regarding VHF/UHF antenna, there are two ways to go: Commercial; or Amateur.

The advantage of commercial antennas is that they are all metal and last longer than fiberglass, and are usually beefier than Amateur metal, so if you want it to last 30 years or more, you may want to consider a commercial antenna. I recently bought a commercial 3-foot Sinclair 900 MHz yagi that was rated for 300 mph or more wind speed and was so sturdy you could use it to pound stakes into the ground with the elements. You can find a wide variety of commercial antennas at www.tessco.com but you might have a hard time finding one listed in the catalog for the Amateur bands. However, if you call a manufacturer, they will usually make one for you.

The other alternative is to use amateur antennas. Their advantages are: about one quarter the cost of commercial, can be multi-band, and are usually much less weight. I suspect they last about 20 years.

At our one-story Talbot EOC/911 Dispatch Center, we have a small 100 foot tower so we have to use small light-weight antennas (as opposed to our commercial 30 foot tall club repeater antenna on a big tower, which weighs close to 100 pounds). We installed two Comet GP-98 tri-band antennas at the EOC so we can run up to 6 radios off 2 antennas and transmission lines to keep tower weight and wind loading to a minimum. The problem with the GP-98 is they are so long they really should have fiberglass top support arms installed to keep the top from wiggling and snapping off in a storm, but ours are close to the ground and protected from wind by nearby trees in the neighborhood and have worked fine for 5 years and counting. We have one installed at 50 feet AGL and one at 100 feet AGL on the tower. We get a very slight hum on a second 2-meter radio speaker when transmitting 50 watts from the first 2-meter radio (both radios are Kenwood TM-D700A) on the other antenna, so I would keep verticle seperation of same-band VHF antennas at 50 feet or more. We have no problem with desensing at that distance, which would be a much more serious problem. And we have no problem with hum if we turn down to 35 watts. Since Talbot County is flat, we can hit a UHF or VHF mobile with a 5/8 mag mount antenna anywhere in our county from the lower tower antenna just 60 feet above sea level (50 feet above ground level). We can communicate to our shelters on 5 watts with GP-3 antennas at the shelters. Our backup plan is to raise a GP-3 on a 35 foot portable tower if we loose our EOC tower or antennas in a storm.

If you only need UHF/VHF I would highly recommend the Comet GP-3 which is only 6 feet tall and has a much higher wind speed rating. I use them at all our shelters and even have one at home.

The reason we have 2 tri-band antennas at our EOC is to be able to use 5 radios at the same time:

VHF-  voice radio to MEMA regional net
VHF-  voice radio for our local Talbot County RACES net
UHF - APRS to track our mobile units
UHF-  Winlink to local Talbot shelters, hosp and health dept
1.2 GHz - video, but we don't really use this much

In retrospect I would use a GP-3 instead of one of the GP-98 because I think it is more likely to survive wind and ice storms.

Regarding HOW we install our antennas: We feed all our antennas with 1/2 inch hardline (Andrew LDF4-50) from the radio patch panel in the radio shack, through the building, to the antenna on the tower. Do not use anything more lossy than LDF4-50 for UHF. If you have over 200 feet of total coax distance, or need every bit of power possible for UHF/VHF and higher frequencies, you may want to upgrade to 7/8 hardline (Andrew AVA5-50) on the tower. We also install polyphasers within 18 inches of entering the building per Motorola R-56 grounding specifications so we don't invalidate the warranty on the public safety radio equipment in the building. Although this is not usually done commercially, I suggest buying polyphasers that are male on one end and female on the other so you can remove the polyphaser and reconnect the transmission lines together temporarily for troubleshooting (the alternative is to temporarily use a F-to-F bulkhead connector for testing for a bad polyphaser). Commercially, only Polyphasers are used for lightening protection. They can take many lightening hits and keep working. In 10 years over dozens of tower sites, I have only ever seen one fail. Polyphasers are grounded to the Master Ground Bus using 6 AWG green jacket copper stranded wire. Coax should be grounded to the tower at the antenna, every 100 feet down the tower, at the base of the tower, and at the cable entry port prior to entering the building. 

When mounting a vertical antenna on a tower, you will need to install a horizontal mast or a standoff bracket to hold the antenna. For the light weight amateur antennas, you can just use a crossover clamp to hold a horizontal mast (pipe) to a tower leg, and mount your antenna vertically on the end of the horizontal mast so the antenna stands at least a wavelength or two away from the tower. For heavier antennas, and to do it "properly", use a standoff made for mounting to a tower leg. Always mount to tower legs or you void the warranty on the tower.

If you don't have a tower, I suggest mounting a 10 foot tall 2-3/8 inch mast pipe to the building wall up high using a wall mount kit, and mounting your antenna on the mast so the bottom of the antenna is above roof level. Some antennas will not mount on pipe over 2 inches, in which case you will need to use 2 inch pipe for your mast which is not as stiff, or what I do is mount a section of 2 inch pipe on your larger mast using pipe-to-pipe standoffs. Don't ever puncture a roof. They are very difficult to waterproof permanently. A wall penetration can be caulked tight with exterior caulk no problem. When you run your coax through the wall, create a drip loop, anchor the coax to the mast and wall so it can't wiggle in the wind, and run it through PVC pipe in the wall so the wall does not chafe the transmission line over time.

Alternatively, you can use non-penetrating roof mounts for your mast, but they will need to be moved periodically for roof maintenance, so I don't like to use them, especially for directional antennas that need to be aligned. They also require you haul a bunch of cinder blocks up to the roof to anchor them.

Transmission line is secured to the tower using hangers every 3 feet. Most towers have cable ladders running down a face for securing transmission line. If the cable ladder has 3/4 inch holes, use the Snap-in Hangers which is the most common method. If the cable ladder is already full, you will have to stack on top of an existing transmission line using stackabe snap-in hangers.

If you tell me what type of connector is on your antenna, how high you want it mounted, and what band(s) you want to pass through the polyphaser, I can put together a parts list for you which will include both manufacturer and Tessco part numbers. I keep all that in a text file I use frequently.

If you put out the work for competitive bid, you can probably get two or more antennas installed by a tower crew in a day for about $2,000 for labor and$ 1,000 for parts. Maybe a little less for wall mounts. Prices vary considerably, so get lots of bids.

For our HF antennas, we are considering installing them as inverted-V, with the high point near the top of our tower at about 100 feet AGL.

In retrospect, I think my email is a good start to a book on installations. Hope you found something in this email useful. Good luck.

Brett Hammond
Talbot County RACES Officer
410-829-6749 (cell)

Sent via DROID2 on Verizon Wireless

-----Original message-----
From: Bob Long <rjlong61 at myactv.net>
To: RACES MRIC LIST <mric at mailman.qth.net>
Cc: Dave Iseminger KB3ISI <Comm503 at aol.com>
Sent: Fri, Jul 8, 2011 21:45:56 GMT+00:00
Subject: [MRIC] A little help

We have just purchased radios for our EOC.

A question has come up about where and how your antennas are installed 
at your EOC.

I wish I could be more specific about what our communications 
department  is looking for, but this is the best I have right now.

FYI we are installing 2 VHF/UHF FM voice radios, 1 HF Voice, one VHF 
Packet/Winlink radio and 1 HF Winlink.

Due to the limitations on available multiband radios we have gone with 
the Kenwood 2000  - not one that was recommended, but one that I have 
been told is in use in quite a number of installations.

Bob Long, KD3JK
RO WASH
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