VHF transmissions on UHF antenna

[CaptDan]

I am just asking the technical question, I understand there may be legal "issues" about specific equipment being approved or used on a particular band. The question I am asking is if my vehicle has a antenna designed for GMRS (UHF) frequencies and on occasion it was used on MURS (VHF) frequencies. Would it even work ? Damage antenna or equipment ?

For example, a group people are going somewhere together, like the beach. It's crowded, lots of people, kids with their roger beep radios. My buddy, a licensed Ham, says here use my walkie talkie, we're going to use MURS instead of GMRS because of all the kids on the radio. I understand his walkie talkie may not be 100% legit, ok maybe it's 0% legit for use on MURS because it's 5 watts rather than 2 watts. If we can just move past that for a moment, and get to the technical part. If I disconnect the antenna from the GMRS radio installed in my vehicle (VHF) and connect that antenna to his walkie talkie programmed for the 5 MURS frequencies (VHF) and then transmit, first is will it damage anything? Radio ? Antenna ? Second will it work? Will it actually do any good, connecting it to a UHF antenna mounted outside the vehicle.

Would I be better off just using the walkie talkie with the VHF antenna installed on the radio?

I am not installing a dual band antenna on my truck because I am migrating away from anything not legit, to a GMRS approved radio mounted in the truck. a GMRS antenna. Just being honest here, there may be times I know I'm going 55 in a 50mph zone, just as I know if I hook use his 5 watt walkie talkie on a 2 watt frequency, neither are legit.

Bottom line - the basic question without all that jibber jabber is, will a UHF radio work connected to a UHF antenna ?

 

[mmckenna]

A UHF antenna will likely not be resonate on VHF. Too short and not going to work well. SWR would be high. Most commercial radios will throttle back the RF power if the SWR goes high, but some may not. I'm not sure if the hammy/CCR stuff will.

So, high SWR. It's not good for the radio, the RF amp will heat up and can do damage. It's not a sudden mushroom cloud the second you key up, it's a heat build up thing. The antenna isn't going work well, and range will suffer.

What you -can- do, if you really expect to run into this issue is use a VHF quarter wave antenna. It's longer than UHF, but will act as a 3/4 wave on UHF and provide acceptable SWR on UHF. I did that for years with a dual band hammy radio and it worked fine. Checked SWR and it was low on both VHF and UHF. Swept the antenna with a monitor and it showed just fine. Radiation pattern gets wonky and less than ideal, but it works fine for some of us.

As for your ham buddy, remind him he's a ham…..

 

[Omega-TI]

At first glance I would not recommend it. Now there are 1/4, 1/2, 3/4 and full wave antennas and I'm not sure MURS and GMRS line up like they do on the amatuer radio bands like 20, 40 and 80 meters. You could check out an online antenna calculator to see, but if your standing wave ratio is off you could theoretically damage the output on the radio.

 

[MTS2000des]

It will work poorly and the high SWR will eventually cause the transmitter power amplifier to burn out.

 

[prcguy]

There are no restrictions on antennas, you can connect a completely wrong antenna to your radio and nobody will care. With that out of the way you want to use a proper antenna for the frequency range that has a good VSWR and radiation pattern. You generally cannot use a UHF antenna on VHF, that just doesn’t work. You can use a 1/4 wave VHF whip (16in long) on UHF as a 3/4 wave, it I’ll have an ok VSWR and the radiation pattern will be ok.

For the best performance I would recommend a dual band antenna designed for the 151 to 155 and 462 to 468MHz range to cover both MURS and GMRS.

 

[paulears]

Don't forget what an antenna does - takes in the power from your radio and converts it into EMF - tuned antennas convert more electrical energy. Usually power is wasted and converted to heat. Devcent antennas also 'launch' your radio energy in certain directions. The wrong length of antenna element might send 50% where you want and 25% upwards, or downwards - and that of course is 50% of the already reduced power, because the radio has throttled back because it senses some of the power coming back down the feeder, having been reflected by the antenna. The old joke is that a random bit of wire works about as well as a bit of damp string. Your aim, if you have 5W is to get as much of that out into the ether. Antennas are also reciprocal - so losing all that energy on the way out also happens in reverse. A touch less attenuation because few receivers are designed to be mega reliant on a proper tuned antenna - like scanners. Their antennas are always compromised.

 

[CaptDan]

so my follow up question is, I think, how much loss of performance would I experience using a dual band antenna.
The primary radio use would be GMRS, but their might be an occasional VHF use.

 

[mmckenna]

so my follow up question is, I think, how much loss of performance would I experience using a dual band antenna.
The primary radio use would be GMRS, but their might be an occasional VHF use.

If it's a true dual band antenna designed for these frequencies, there wouldn't be any unnecessary loss.

In fact, if you compared it to one radio with a diplexer and two separate (VHF and UHF) antennas, it would be a bit better. Diplexers add a bit of additional loss and cost more.

On the other hand, dual band antennas will have different radiation patterns between the two bands, so all other things being equal, there will be a difference.

Some prefer to have separate antennas where they have control over the radiation patterns.
Some prefer to have a single multiband antenna where you don't need the extra roof real estate or the diplexer.

I'm running Harris XL-200M's in my work and personal trucks and both are connected to one multiband antenna, and it works fine for my needs. I know others on this site that prefer dedicated antennas for each band. Really depends on your own needs and if you have the roof space for separate antennas.

 

[Golay]

We have a local ham that has been running a VHF quarter wave as a dual band antenna for 20 years. He makes the 440 machines good with it.
Never heard him talking about his match. But he is one to check that sort of thing. I suppose a quarter at 2M would be three quarters at 440.
Probably a bit of a cloud warmer. But our machines have great coverage. So there you go.

 

[mmckenna]

I suppose a quarter at 2M would be three quarters at 440.

It is, odd multiples of 1/4 wave. Always worked fine for me. I've swept them before on my analyzer at work. They have a good match at a big chunk of the UHF band.

 

[paulears]

They’re a perfect match and the capture area makes them nice and efficient. My van has twovhf ¼ waves one cut for ham and a slightly shorter one that does marine and business UHF

 

[speedway_navigator]

The left sweep shows return loss for a quad band APX antenna. VHF and UHF are very good for a portable considering no ground plane.
I put a 14" collapsible antenna on my R54 and made the other 2 sweeps from 100 MHz to 1 GHz. The middle is return loss in decibels, the right one is for veterans of the vis wars, thank you for your service.

 

[nd5y]

Now make field strength and radiation pattern measurements for the entire frequency range for each antenna.

 

[prcguy]

I ran a VHF 1/4 wave whip on a test range at UHF and its about 2dB down +/- about 1dB compared to a 1/4 wave UHF whip. In this case I used a 100MHz to 1GHz log periodic as a source antenna and an Agilent FieldFox in S21 mode with amplifier then calibrated 400-500MHz using a 1/4 wave UHF whip then substituted a 1/4 wave VHF whip and the resulting loss is what you see below. Whips were mounted to an NMO trunk lip mount on the hood of my truck. Bottom line is a VHF whip can have a good match at UHF but the radiation pattern at the horizon is not ideal and you will loose some range compared to a 1/4 wave UHF whip.




Anything else I can do for you boys today?