Full vs Half vs Quarter Wave Mobile Antenna & what tune Fr

[Hasaf]

I decided to go with a separate unit install instead of just accessing GMRS using my VHF/UHF. This means installing another antenna on my FJ Cruiser.

 

This is enough antennas to push me over the line, drop the headliner and install NMO mounts instead of the "clamp-On" that I am currently using. This got me to looking at the Larson NMO tunable, a favourite when I was playing with radios more often. Realizing how short the wavelength is, has anyone played with different lengths?

 

The "no ground plane" feature is pointless as it is going to go on a big sheet of steel (the roof). The bit of gain is more valuable to me. I then got to thinking of full wave, I can't think of anyone I know ever using full wave, there has to be a good reason not to, but I am not thinking of any. 

 

The antenna comes pretty long, with the intention of being cut, but it can't be un-cut. Any thoughts?

 

Next question, what is the most common Fr to cut the GMRS antennas to?

[gortex2]

The antenna must be tuned. If your using the larsen go by the cut chart. Personally I'd just order the UHF 1/4 wave. If you do go by cut chart you should see it at about 6"

[mbrun]

I decided to go with a separate unit install instead of just accessing GMRS using my VHF/UHF. This means installing another antenna on my FJ Cruiser.
 
This is enough antennas to push me over the line, drop the headliner and install NMO mounts instead of the "clamp-On" that I am currently using. This got me to looking at the Larson NMO tunable, a favourite when I was playing with radios more often. Realizing how short the wavelength is, has anyone played with different lengths?
 
The "no ground plane" feature is pointless as it is going to go on a big sheet of steel (the roof). The bit of gain is more valuable to me. I then got to thinking of full wave, I can't think of anyone I know ever using full wave, there has to be a good reason not to, but I am not thinking of any. 
 
The antenna comes pretty long, with the intention of being cut, but it can't be un-cut. Any thoughts?
 
Next question, what is the most common Fr to cut the GMRS antennas to?

Good Day.

When I started playing with radio back in the 70s I used to wonder the same thing regarding full-wave antennas. If a quarter wave antenna makes for a good working antenna and a half-wave is better, why isn’t anyone using a full-wave antennas. Well, the simple answers are 1) Antennas are larger than their shorter counter parts(a really big issue at lower frequencies) and 2) through testing it has been determined that they are very poor performers. You can be certain that with so much experimentation and research into radio and antennas for over a century that if full-wave antennas performed even remotely better they would be in heavy use. However, 1/2 wave antennas have been determined to be the clear winner. Oddly enough, even the 1/4 wave antenna is in effect a 1/2 wave antenna in that the ground plane (or ground radials) effectively satisfy the function of the missing 1/4 wave portion. If you search the internet you will find the more technical reasons why the full-wave version performs poorly.


Michael
WRHS965
KE8PLM

[OldRadioGuy]

Seems like you could just use your 440 MHz / 70cm ham antenna with a coax switch.

I've tested several of my hamband UHF antennas on GMRS and all seem to match up well enough.

Vince

[WRMH343]

On 7/5/2021 at 5:08 AM, OldRadioGuy said:

Seems like you could just use your 440 MHz / 70cm ham antenna with a coax switch.

I've tested several of my hamband UHF antennas on GMRS and all seem to match up well enough.

Vince

I only have one external antenna on my home (it is tuned to the ham bands).  Connected to a Ham HT, i get about 1.1 SWR.  Connected to a GMRS HT, I get about 1.7 SWR.  Not perfect, but very useable, and way better than an indoor rubber ducky.

[WRNA236]

The reason a 1λ antenna isn't common is the radiation pattern is starting to develop odd lobes. 

Physically and electrically an antenna has to be some multiple of the wavelength at your frequency but not all multiples produce the same result.  The baseline 1/2λ dipole or 1/4λ monopole (half a dipole where the other half is ground) produces a pattern that's generally uniform and has one primary lobe and has a characteristic impedance that's easy to match with a radio and coax.

We tend to use 1/2λ and 5/8λ monopole antennas because they show gain without a ton of oddity or artifacts.  These lengths aren't difficult to match so they're worthwhile trade-offs. 

Other lengths are difficult to match or practically longer without offering much benefit.  The little bit of gain in the lobes you get with a 1λ over a 5/8λ comes with a couple of significant nulls which will result in fading as the whip flops around.  That means it'll go from strong signal to weak annoyingly, so it's not a preferred mobile configuration.  The upside is a 1λ whip can be matched like 1/2λ. 

FWIW, I use a slightly long for the 2m ham band 1/4λ whip with a diplexer for VHF APRS and GMRS, which means my antenna is actually 3/4λ.  Odd multiples match the same so a 1/4λ for 150 MHz will be resonant (and the same impedance) again at 450 MHz, 750 MHz, etc. just with the increasing number of lobes as the whip becomes several wavelengths long.