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Author Topic: High duty cycle VHF mobile  (Read 7906 times)

Posts: 3

« on: June 19, 2011, 05:12:52 PM »

I'm looking for opinions on a high duty cycle mobile to use a base for playing AR newsline on the local repeater and for using as a micro node for IRLP.  5-10 watts.

73 de AJ6B
« Last Edit: June 19, 2011, 05:32:20 PM by AJ6B » Logged

Posts: 17353

« Reply #1 on: June 19, 2011, 06:30:28 PM »

Any 25 to 50 watt radio with a suitable low power position should do the job.

You can add an external fan pointing at the heatsink if you are still worried.

Posts: 21757

« Reply #2 on: June 19, 2011, 07:25:13 PM »

A GREAT 2m rig for that:

-Surplus Motorola Micor (very cheap), these are 100W rigs that can handle 100% duty cycle, for repeaters.

-An old Kenwood TR-7400A.  25W rig that can actually produce 25W for hours on end without derating.

-Or as Dale said, a 50W mobile rig operated in low power at 5-10W should survive that for a long time.

LOTS of options! 

I have an old Yaesu FT-736R that runs 25W output on 2m, and I've run it continuously a few times for 3-4 hours at that power and it does not overheat at all.  It has a very big heatsink for such a "low power" rig.

Posts: 5688

« Reply #3 on: June 26, 2011, 08:16:32 AM »

The key here is to *derate* the transmitter. 

That means not running it flat out at full power, but instead setting it up to run some *fraction* of its full power output rating. 

As stated, a 25W radio running 5-10 watts will not only use less power but will generate less heat in the finals, allowing it to function at the higher duty cycle rating just fine. 

Adding a fan pointed at the heatsink can also help out as well. 

Since it is fixed station operation, the ANTENNA choice, plus height above ground is where you get the coverage, not with raw output power but with ERP. 

Over the years I've pressed just about any well working transceiver into such service without problem. 


Posts: 853

« Reply #4 on: June 29, 2011, 01:01:13 PM »

"Surplus Motorola Micor (very cheap)"

Yack!!!  Purchasing crystals, aligning, wacky interface, large footprint cabinet...  Unnecessary effort.  You can snag an old 2-channel MaxTrac at a hamfest for about $20.  The mic jack is an RJ-45 making interfacing very easy.  A 45w MaxTrac de-rated to 10w will run all day long.

Posts: 297

« Reply #5 on: July 11, 2011, 10:14:49 AM »

I was going to suggest the max trac my self Smiley  I have two of them I use.. one in the mobile, one in the shack here..

mine are (were 4 channel) now 32 channel with the full interface option.. I got lucky when I found these.. rated 25w.

the thing about MaxTracs there a couple different versions the GM300 which looks the same, but I don't think has the Maxtrac name on it.. while being a more capable radio, has very limited finals protection.. it 'calculates' finals temp by how long you've been transmitting and shuts TX down and a pre-determined interval.. kind of a pain for what you are doing.
the 'lesser' maxtrac model, actually uses a temp sensor in the finals, and will continue to Tx as long as the temps remain ok.. and then as I understand it, will gradually throttle tx power down to whatever the radios minimum is.. and if you turn it down from it's maximum output of 25w down to about 10 like you are saying, you will have no problems.  just programming a maxtrac can be a bit of a pain. the interface box isn't too hard to find, but it requires a SLOW computer running REAL MS-Dos, (dos box on a newer machine won't work)
it's gotta be MS dos 6, 7 (win95/98)
and it requires a real RS-232 port,  USB to RS-232 won't work either.


Posts: 12371

« Reply #6 on: July 11, 2011, 11:26:00 AM »

Any 25 to 50 watt radio with a suitable low power position should do the job.

You can add an external fan pointing at the heatsink if you are still worried.

If high duty cycle is needed, aux cooling with a small fan will keep radio cool. Forced air cooling should always be used in high output duty cycles. I would suggest getting a 50 or 60 watt mobile with a mid power setting of about 25 watts or so as that is only 3db down from 50 watts and with aux cooling it could handle key down for a very long time without problems.

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 9930

« Reply #7 on: July 22, 2011, 11:01:17 AM »

look at the yaesu 1500. a nice little 2 meter rig. and solid as a rock, and built like a tank.  it has no built in fan on it, the whole case is a heat sink.  my suggestion is you can mount a small ( 2 or 3 inch) computer fan on it and turn on the fan when using it for a long transmission. great little rig. ( or perhaps mount it where a vent will blow on it)
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