Cutting 27mhz Fibreglass whip to 400mhz

[NSWESP]

I have a pair of old 27mhz fibreglass whip antennas that i need cut to 416 and the other to 468mhz. I was wondering what length i need the internal wire to be for these and if i should wind it into a coil?

These are both only needed for a short period of time and in the next few days so ordering a new one is not worth while.

[mancow]

Roughly 6" but do you know if the internal conductor is wound or straight?

[NSWESP]

It has a fibreglass rod and the wire is wound around with a loose coil right at the bottom and then the wire is wound very loosly around to a coil close to the top of the antenna. Very similar to the DX120FW on this page (Not far from the top) Untitled Document (its not this exact antenna)

[NSWESP]

Also is that 6 inches for the 416mhz or 468mhz? Im looking to cut 2 seperate antennas one for each. Am i correct in thinking that the one cut to 468 will work fine on 416 but not the other way around?

[n4yek]

Total length including the mount from the ground plane surface:
416 MHz = 5.33"
468 MHz = 6.0"

[rfradioconsult]

Go to your hardware store and buy a 3/8 x 24 bolt 6 inches long and screw it into your mount; for your purposes there will be not difference between a 6 or 6.3 in antenna.

[prcguy]

If you have a ball mount that should be more than large enough to resonate at both UHF frequencies.
prcguy

[NSWESP]

I'm using a standard raiser as my mount no ball mount. Do those measurements make sense? Shouldn't the smaller the freq the longer the antenna

[jackj]

Originally Posted by NSWESP

I'm using a standard raiser as my mount no ball mount. Do those measurements make sense? Shouldn't the smaller the freq the longer the antenna

Higher frequencies have a shorter wave length. This means that the "smaller" the frequency, the longer the antenna if "smaller" means lower. Use this formula for figuring antenna length:

L = Antenna length in meters
F = Resonant frequency in MHz

L = (300 x 0.94) / (F x 2)

This will get you pretty darn close to the proper length of a straight wire.

Jackj

[n4yek]

Originally Posted by n4yek

Total length including the mount from the ground plane surface:
416 MHz = 5.33"
468 MHz = 6.0"

Originally Posted by NSWESP

I'm using a standard raiser as my mount no ball mount. Do those measurements make sense? Shouldn't the smaller the freq the longer the antenna

My mistake,
416 MHz = 6.75" (was looking at another totally unrelated formula answer when I typed it the first time)

[kb2vxa]

I really wouldn't go there, lead dressing in HF mounts isn't given the same consideration it is at UHF. The long and short of it is any exposed portion of the coax center conductor and the pigtail ground connection become part of the antenna making the results extremely unpredictable.

[prcguy]

The 6" bolt is a good idea and a 3/8 diameter radiator will be very broad band to cover both freqs if you get the length close.
prcguy