Can a 27Mhz CB Aerial be used for a UHF Radio in a emergency?

[rob_macca67 Avid PP Poster! Find all posts View Profile Close rob_macca67]

Guys....
Not knowing much about Radio aerials, etc. I was wondering if u could use an old 27Mhz CB Radio Aerial as a replacement for a UHF Radio Aerial in an emergency and if there would be any damage done to the UHF Radio?

Currently I've got an elevated Feed UHF Aerial (Uniden AT870) for our car and I was wondering if for some reason the Fibreglass screw-on aerial snapped off where it screws on top of the elevated feed whether I could as a temporary measure, just simply screw on an old 27Mhz CB Radio aerial that I've had stored in my shed for years and whether this would cause any damage to the UHF itself?
I know we are talking about different frequencies here (27Mhz vs 470Mhz) but for an emergency I thought it might be ok.....

If I did this what would expect to happen..... poor transmitting/receiving ??? radio damage ???



rob

[vk2ss55 Senior Member Find all posts View Profile Close vk2ss55]

Rob I wouldn't use the 27 ant on UHF as it will have what's called a high VSWR which can damage the radio . But hey............if it's an emergency well you've got to do what you can !
I'm a ham radio operator so on UHF which is 477mHZ a basic 1/4 wave antenna is about 150 mm long so you could just stick a screwdriver this length onto the base and you'd be away .
Cheers
Ross

[rob_macca67 Avid PP Poster! Find all posts View Profile Close rob_macca67]

Thanks Ross, that's exactly what I needed to know. So being a 1/4 wavelength, then what about lengths of 300mm, 450mm and 600mm... any of these lengths OK ??

cheers

rob

[Bluebox Lurker Find all posts View Profile Close Bluebox]

Wavelength is equal to (speed of light) divided by (frequency). So for a quarter ground plain unity gain type antenna would be one quarter of the full wavelength. So that would be equal to (299792458m/s divided by 467900000Hz) divided by 4. I used the Channel 20 simplex TX frequncy to do the calulation as its in the centre of the band. Unfortuately the groundplane is not a perfect shape. (ie a Prado) So this is why antennas should be trimmed or turned to the correct length. Also the length of the transmission line can have a bearing. In worst case stub matching needs to be employed at the base of the antenna. Most UHF antennas seem to employ "gain". 3db, 6db are common. The antenna is loaded with tank circuits turned to applicable frequency to concentrate the radiation donut to a narrower beam. I think the little stainless steel whip on my car is a 5/8wave (groundplane independant).