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Commercial Radio Antennas Please keep discussion related to professional, commercially used antennas and antenna systems for the two-way radio industry. Topics for the use of these antennas on amateur bands are accepted here.

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Old 10-18-2013, 1:42 PM
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Default Low Band Antenna Issues

I am trying to get my low band radio and antenna system up and running but I am having issues with, what I believe with my antenna.

Radio: Motorola CDM750

Freq: 33.5-34

Antenna: Maxrad MLB3000 without spring

I tuned the antenna following the directions for proper length. I hook up my Bird meter and read forward wattage of 43 watts (Exactly where I have the mobile programmed for) Reflected wattage is 23...

I then tested the radio directly to a dummy load and get the same 43 watts forward but then 0 reflected.

I tracked down another antenna, though, with a spring and tested and got around 25 watts forward but 2-3 watts reflected... Have no idea why forward was way lower so I switched back to my antenna and went back to 43 watts forward.

I then got out my CDM1550LS+ UHF and put a UHF antenna on that mount and tested that coax and NMO system and got the proper forward wattage and 1 watt reflected.

So with all that testing, I believe that the radio is ok, coax and NMO mount are ok. Agree? Am is missing something? HELP PLEASE!
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Old 10-18-2013, 2:17 PM
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Did you check to see if you had the correct low band coil? It is possible you have a bad piece of coax or mount. Put a dummy load on the antenna mount (use an NMO to SO-239 adapter or whatever your type of connector) and see if the reflected power is still high. That would lead you to a bad or wrong coil.
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Old 10-18-2013, 2:33 PM
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33.0 - 34.0 MHz isn't an amateur radio band. Not sure why you'd be transmitting there?? Or should this be in the commercial radio antennas forum?

That sounds like the SWR is high on the antenna. If you have 43 forward and 23 reflected, something isn't good on the antenna. Do you have another radio you can test the antenna on and check the SWR from both radios? Also, where is the antenna mounted when you're testing it?
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Last edited by WB4CS; 10-18-2013 at 2:47 PM..
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Old 10-18-2013, 7:56 PM
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How do you have the antenna mounted, in low band you have to pay attention to how it is mounted.
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Old 10-18-2013, 11:26 PM
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Whip length is very critical with that type of a coil setup. An inch can make all the difference. I would see if you can borrow an analyzer (an HF ham one should work for that range) and see if it has good SWR at any point, then trim the whip until it is on freq. Be advised that your bandwidth of good SWR is going to be narrow with that style antenna.
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Old 10-19-2013, 10:01 AM
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I probably should of been put the thread in the commercial section. It's for fire company communications. The antenna must be bad because I found a spare antenna and now I'm getting 43 watts forward and 2 reflected. I guess sometimes you can get bad equipment new... Thank you everyone for the help!
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Old 10-19-2013, 8:50 PM
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Quote:
Originally Posted by jluisi86 View Post
I probably should of been put the thread in the commercial section. It's for fire company communications. The antenna must be bad because I found a spare antenna and now I'm getting 43 watts forward and 2 reflected. I guess sometimes you can get bad equipment new... Thank you everyone for the help!
Let me state what I did not make clear: the antenna cutting chart only gets you close and it has to be fine tuned once installed. My money is that the whip needs to be cut more precisely. But you need to have multiple transmit frequencies to check this, or an analyzer. But this would not be the first Maxrad I have seen with a bad coil either.
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