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[N5RZX]

Got a question on a mag mount antenna.. I’m trying to cut a wire for my Larsen mag mount to the middle of the 2M spectrum. Now, correct me if I’m wrong, but a 1/2 wave 2m doesn’t require a ground plane, correct? Well, reason I’m trying to make one, is my SUV has a glass roof, with a small section of metal at the rear that’s about 8 inches wide and spans the length of the roof, about 5 feet. That’s where my mag mount is to go. Now, to simulate a metal ground plane, a long baking pan was used for the metal, figuring it would approximate the theoretical un-needed ground plane. Well, tried tuning this antenna starting at 41” of whip (bottom of mount to whip is about 1.5”) and cut down in 1/4 intervals. I got down to 37” and still my SWR meter pegs out, and power output of my radio (75w) is showing about 30-40 watts depending of frequency. So I know the meter is telling me SWR is bad off. So can someone tell me why this is occurring? I know not having a true ground plane won’t give me the best SWR, but something’s gotta be up with this. I’ve check for antenna shorts, wiring, etc. it is an NMO mount, and I really don’t have any other options for mounting.  Now I do have a miniature 1/4 wave mag mount that does work on the vehicle but is not the best. But it’s SWR is 2.0 and I can live with that for its use, but was trying the 1/2 wave figuring there just wasn’t enough metal on the vehicle. Any suggestions?

[W1VT]

It is likely that the NMO mount has no matching network.
A half wave antenna is high impedance.  On HF a 49:1 unun is used for matching.
You need some sort of matching network.

[WB8VLC]

Zach got the answer a microsecond before my post so he gets the credit,

Is this just a half wavelength of wire going directly to the NMO connector without the required matching network?

If true then you still need the Larsen or similar base matching coil which matches the half wavelength long whips high impedance to 50 ohms.

As an example just a half wave length piece of 12 guage wire (39 inches in my model)  which is endfed, has an impedance in free space around 1400 ohms real and 3000 ohms capacitive  or ~1400-J3000, hence the off the charts VSWR measured in a 50 ohm system.

[WB6BYU]

A common matching network for a 1/2 wave
whip on 2m is a coil of perhaps 5 turns or so,
tuned by a small variable capacitor across it.
One end of the coil goes to ground, one to the
antenna, and the coax center conductor is
tapped about 1 turn up the coil (adjusted for
lowest SWR along with the variable capacitor).

I’ve modified 36” CB magmounts for this
purpose by changing out the loading coil in
the base and adding the capacitor.


A quick check would be to stick an 18” wire
in your base and measure the SWR on 2m.
If it is low, then the base doesn’t contain
the matching required for a 1/2 wave whip.

[N5RZX]

Ok, I didn’t realise I needed a specialized base, reason being, I have an ordinary NMO mag mount with a single, 40” wire cut, in the same way I’m trying to do now. It sits atop a metal in-ground tornado shelter and has an almost 1:1 SWR. So in order to have this half wave work I’ll need an NMO mount that’s specific to 1/2 wave antenna?

[N5RZX]

So is there a matching network in the part here that has the brand name on it?

https://www.ruggedradios.com/products/vhf-antenna-kit-with-half-wave-ngp-antenna-nmo-mount

[WB6BYU]

Most likely the base contains some sort of
matching network.

[KH6AQ]

Ok, I didn’t realise I needed a specialized base, reason being, I have an ordinary NMO mag mount with a single, 40” wire cut, in the same way I’m trying to do now. It sits atop a metal in-ground tornado shelter and has an almost 1:1 SWR. So in order to have this half wave work I’ll need an NMO mount that’s specific to 1/2 wave antenna?

Something is wrong here. A 40" ground plane in the 2 meter band should have an SWR of over 20:1.

These antennas at DX Engineering will work:

Larsen VHF/UHF Mobile Antennas NMO150C

Larsen VHF/UHF Mobile Antennas NMO150B

[N5RZX]

Most likely the base contains some sort of
matching network.

Would ohm meter be able to distinguish whether it has or not?

And I take it not the magnetic part, but the NMO part between the whip and the magnetic base?

[N5RZX]

Ok, I didn’t realise I needed a specialized base, reason being, I have an ordinary NMO mag mount with a single, 40” wire cut, in the same way I’m trying to do now. It sits atop a metal in-ground tornado shelter and has an almost 1:1 SWR. So in order to have this half wave work I’ll need an NMO mount that’s specific to 1/2 wave antenna?

Something is wrong here. A 40" ground plane in the 2 meter band should have an SWR of over 20:1.

These antennas at DX Engineering will work:

Larsen VHF/UHF Mobile Antennas NMO150C

Larsen VHF/UHF Mobile Antennas NMO150B

Well, according to QSL.net, 468 divided by 146(mhz)= 38.5 inches, so mathematically it should be between 40 and 38 inches.

[WB8VLC]

You need the larsen NMO150HWBCO bottom piece/ half wave nmo mount coil assembly which cost 13 dollars at HRO.

[N5RZX]

Thank you all for your help. I understand it now! 

[N0GV]

A vertical 1/2 wave is no better than a 1/4 wave with a ground plane for vhf and, properly done the 1/4 wave 2m antenna can be made into a dual bander (445 MHz) with the addition of a phasing coil... 1/2 wave over 1/4 wave... or stacked 3/8 waves.... Many commercial examples exist and they are all pretty rugged.....