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Improving ufh voice radio

Improving ufh voice radio

Improving ufh voice radio

I am using a motorola cdm750 voice radio in a carbon fiber race car.  The frequency range is 450 Mhz to 470 Mhz and I am currently using a 3db gain antenna and the power is up to 25 watts on the radio.  My problem is that as soon as I lose line of sight the radio is nothing but static pretty much.  The odd thing is that I am using a data telemtery radio in a 440 Mhz range and I do not lose the transmission when I lose line of sight.  Any suggestions to improve this situation?  Also would it be helpful to clear up any static if I used a noise filter on the inline power?

RE: Improving ufh voice radio

Q1- Where is the master/repeater antenna located? I assume the antenna is located on a tower somewhere in the race field to provide adequate coverage?

Q2- Where is the antenna located on the car?

Q3- What model of antenna do you have?

Q4- Do you have a ground plane under the antenna mount?

It is likely that you have a mobile antenna that requires a ground plane. These are relatively easy to install. If this is a roof or trunk mounted antenna, you can install a self adhesive aluminum ground plane under the surface of the ceiling/trunk, then ensure there is good electrical contact to the antenna housing.
You can also try a no ground plane antenna; Antenex has a 3dB gain, low profile stealth design antenna that requires a Motorola style mount, P/N- ETRA4503.

I would install a filter on the power to the radio, every little bit helps.

RE: Improving ufh voice radio

The master repeater is located in the pits of the track, not centrally located at the track.

RE: Improving ufh voice radio

The Motorola HAE4011 roof mount 3.5 dB antenna is made for low launch angles, which sounds like what you want for the non mast mounted repeater antenna.  Either way, a ground plane is impartant, as BrianR said, and is also part of the antenna itself and not just a nicety.

RE: Improving ufh voice radio

25 watts at UHF should go a lot further (miles) than 'just out of sight', even with a poor rubber duckie antenna. Based on that, it seems like something is seriously wrong.

Has anyone checked the current antenna system? Measured the VSWR? ...not for purposes of tweaking to perfection, but simply to confirm if things are connected properly at all.

RE: Improving ufh voice radio

I agree.  25W into an poorly shielded dummy load is good for a mile or two.

RE: Improving ufh voice radio

The VSWR was checked with the car not started @ 1 to 1.3 depending on the area top, bottom, side to side.  I am thinking that I am getting some kind of interference from the car when it is running around the track at high rpm's.  The telemetry radio is mounted right ontop of the voice radio, do you think this could be causing some of the interference as well as possible the ECU which is about 6 inches away.  Also it is mounted about 4 inches away from the battery of the car, could there be interference coming from the power, I am planning to put an ENF(electronic noise filter) right before the radio power input to try and filter some of the noise coming into the radio.  

Any additional ideas?

RE: Improving ufh voice radio

"VSWR...depending on the area top, bottom, side to side."

Huh? Checking VSWR means inserting the appropriate meter into the coaxial cable (using a coax jumper, preferably down near the radio) and simply taking the reading. What exactly do you mean by "depending on the area.." ??

Also, starting the car shouldn't affect it. If it does, then the connections are loose.

By the way, 1.3:1 is perfectly acceptable if that is an accurate reading.

Might as well check the transmitted power while the meter is connected. Usually just turn the slug around or flick a switch. What type of VSWR meter was used?

A high electrical noise level in the car would not normally be expected to affect the outgoing 25 watt transmitted signal other than any noise you might hear on the transmitted audio (when the car is within radio range). In other words, having someone whisper in your ear doesn't normally affect your ability to shout.

On the other hand, a high noise level in the car might certainly make the radio deaf, even if the noise is rejected by the FM discriminator (in other words, the radio can be deafened by a high noise level even if you don't hear much noise on the driver's headset).  But, most normal electronics doesn't generate much noise at frequencies as high as 470 MHz.

RE: Improving ufh voice radio

I think, the voice 3 inch antenna energy is getting sucked up by your composite fiber, which also makes the VSWR look good. It's like adding a resistor to your antenna. The Telemetry radio looks like it's attached to a metal oval shape. The voice antenna can't be 3 dB gain, a bit o marketing going on there, maybe 3 dB directivity, but probably -3 or -5 dBi gain.

To evaluate if the carbon is sucking up the energy, try taking Reynolds wrap aluminum foil, circular shape, 15 inch diameter and add it to the base of the voice antenna, oriented horizontally. Alternatively, take 4 wires (each 10 to 20 inches long, awg #24 or thicker) with alligator clips and attach them to the voice antenna base, spread them out horizontally. Or just strip them and wrap them around the antenna base and tape them down. If your range goes up alot, it's probably the carbon fibre as the culprit.

Can you also see what your signal strength is when your receive Voice. Have someone with a spectrum analyzer compare the voice and telemetry signal strengths. If the antenna is the culprit, you'll see big differences just standing in front of your vehicle.


RE: Improving ufh voice radio

Regarding antenna position wrt other things: A second UHF antenna about 6" away is an issue as 6" is approx half a wavelength and so a directional antenna results.

A battery nearby is unlikely to be any issue other than shielding, nor will a real bad VSWR prevent transmission over a mile or so with a 25W TX, provided some sort of antenna is connected.  The proximity of the ECU might affect reception due to EMI.

What is the problem again,  is it transmission from the car to base, or received signals at the car?  A source of local interference could block reception, and an antenna that is radiating straight up in the air could block things both ways.

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