Repeater antenna for mobile use

[baltimorecs]

I purchased another GR1225 UHF repeater and this one does not have a duplexer. The others I have all are tuned to a specific frequency (we run between 450-470 MHz) and have magnet mount antennas on a metal ground plane. This one I wanted to use to take to different training sites and I need to be able to program it on the fly. When we go onsite, I get an STA to use the repeater and our portable and mobile radios, but the STA states that we can use the requested frequencies on a secondary, non-interference basis. So even though I check the nice database here for licensed frequencies, we still run across others using our requested frequencies on occasion--some times necessitating a change in frequency pairs. To that end, based on the suggestion of some members here, I bought two antennas and a 30 foot mast kit. I have done a lot of research on antennas and plan to separate the antennas vertically, but was wondering if I could ask a few questions? The repeater is for use during training events in wooded or hilly areas. It is a supplement to the cell phones that we are given as we find cell coverage in some of these areas spotty at best. We would use this repeater 4 to 5 times per year, 2 to 4 days each time.

If anyone would be willing to help me I would greatly appreciate it. Thank you!

- The Heliax cable that was recommended to be seems to be too stiff to coil and pack for transport (some times by aircraft). Is LMR400 ok for a portable installation with separate antennas? It would run approximately 50 feet from the receive antenna and 40 feet from the transmit antenna. Using the calculator on timesmicrowave.com I would have approximately 1.6db loss for the 50 ft run, which is acceptable to me.

- The antennas are Tram Browning BR-6155 5/8 wave stacked antennas for 450-470 MHz. We don't run training in inclement weather so they aren't going to have to take much of a beating, but do they have to be grounded? The repeater can be grounded, and I don't believe these antennas require grounding, but I cannot confirm that.
Tram/Browning BR-6155 [Tram/Browning BR-6155] - $79.95 : The Antenna Farm, Your two way radio source

- I plan to stack the receive antenna on top (lets say 30 ft) and the transmit antenna 10 to 15 ft below for vertical separation. If we were to use HT's I figure this would give the best reception to the repeater antenna. I just could not find a best practice for antenna placement.

Please bear in mind that this is all out-of-pocket for me as I am not reimbursed for this so I am trying to keep cost in line with acceptable performance. It does allow me to expand my inner geek to radios however.

 

[prcguy]

The best advice I could give is get licensed for several consecutive frequencies and get a mobile duplexer as they can usually cover 100KHz to 150KHz of frequency spread without retuning. Using two antennas without any band pass cavities as you intend usually results in frustration and poor repeater performance.

Having maybe 5 consecutive pairs of repeater freqs should allow you to find a quite one in whatever area you operate and it sounds like radios would have to be programmed at each site anyway so why suffer with headaches caused by repeater desense?
prcguy

 

[rpurchases10]

What he said. It's fairly inexpensive to obtain an FCC license for a pair of itinerant (nationwide use) - or maybe x2 in case of local use of your "primary" pair -- of frequencies - this way your equipment is "pre-programmed" including a quality duplexer to permit the use of a single quality antenna as well.

FB-2i (mobile relay aka: repeater / the "i" indicating "itinerant") and "MOi" - mobile "itinerant".

This is how many rental companies handle their licensing - especially if they do not know the exact area of operations for each rental, etc.

Make a bunch of different length "coils" of Andrew "Superflex" to provide for different temp installs and you are good to go .

The freq coordination / lic prep should be around a few hundred bucks since they are essentially "uncoordinated" and your fcc license (for as many freqs as you require) is just $210 for a 10-yr length of time. (if .gov - fed/state/municipal agency - n/c on the fcc side)

Good luck !

 

[rpurchases10]

oops. forgot to mention - Do NOT use LMR-400 or LMR-600 if running a duplex repeater. Go straight to a heliax/superflex type of hardline and, to "jumper" to the antenna(s) / duplexer(s) / radios(s) / repeater(s) --> go with a good quality brand of RG214 (Belden etc)