Mobile milair antenna

[BruceB]

I'm in search of a nmo milair antenna, any suggestions would be greatly appreciated.

 

[DPD1]

For UHF? A single 1/4 wave whip at about 9" and you're done. There's other things that you can do, but it's more complex and I don't believe any manufacturer sells them for a consumer oriented price. I've been working on various designs for a while, but haven't settled on anything yet. One would be a combination UHF/VHF Air model.

Dave
http://www.dpdproductions.com
- Custom Scanner, Aviation, MURS, GMRS, Marine & Ham Antennas -

 

[ka3jjz]

While I don't know of anything mobile specifically cut for the milair bands, I do know that there are a few widebanded designs out there that include these bands. One of the members on the Maryland milair thread uses an antenna from Comtelco (probably one of their quarter wave wideband designs) and the mobile version of the NilJon seems to have a following as well. These links, and others can be found on our Scanner Antennas wiki article. Don't forget to check the EHam antenna reviews page to see if someone has discussed the antenna in which you're interested, as well as the wiki article above to see if there's any reviews to be found.

I also understand that triband ham antennas - I think Diamond sells one - that work on 144/220/450 also work pretty well as a milair antenna.

73s Mike

 

[BruceB]

For UHF? A single 1/4 wave whip at about 9" and you're done. There's other things that you can do, but it's more complex and I don't believe any manufacturer sells them for a consumer oriented price. I've been working on various designs for a while, but haven't settled on anything yet. One would be a combination UHF/VHF Air model.

Dave
http://www.dpdproductions.com
- Custom Scanner, Aviation, MURS, GMRS, Marine & Ham Antennas -

just courious Dave, how did you come up with a length of 9"?

 

[SkipSanders]

Wavelength = 300/frequency (in meters) 300 MHz center of the milair band (close enough)

So, 1/4 wave whip is 1/4 meter long. Allowing for a .95 lengthening factor due to thickness of the antenna in wavelengths, this produces a 9.3" whip.

If you honestly wanted to create a 'broadband' version, you'd build it of very high diameter tubing, say 6" dia. copper. You'd need to look up the actual formulas for adjusting for element thickness, but this would widen the antenna out quite a bit. In practical terms, for receiving, it really doesn't matter that much. And it'd be a hum-dinger of a WHAP if you hit something with that mounted 6" dia tube, too!

 

[ka3jjz]

I'll bet he used the basic 234/f(mhz) formula - substituting 300 for the freq results in .78 foot, which is roughly 9 inches and change. That might work OK for UHF, but be pretty lousy in the 138-144 mhz range.

73s Mike

 

[DPD1]

Yes, but using the math is always just a starting point, because every antenna and situation is different. I've trimmed a few different mobiles to the band with a field monitor though, and the middle came out to about 9" on all of them. Which is obviously no big deal with a few MHz one way or the other anyway, since the band is so wide. But because the ground plane is inherently wide in bandwidth, it does surprisingly alright in it's simplicity. A 5/8 would give more gain in the middle, but less bandwidth over such a large band. The government uses fat dipoles for mobile use, which also does better on neighboring bands... But as mentioned, then you get into physical issues, and a mag mount isn't going to hack it in most cases. Unfortunately it's the physical limitations in mobile stuff that limits performance.

Dave
http://www.dpdproductions.com
- Custom Scanner, Aviation, MURS, GMRS, Marine & Ham Antennas -

 

[n7jei]

Hi all, I tried the 9 inchish antenna in the center of the roof of my truck (drilled hole mount, not mag) and wasn't very happy with the results. Also tried a ham 220 quarter wave and wasn't happy either. I changed to my VHF air band antenna (~20 inchish or so) and had VERY much better results, and I could listen to VHF at the same time on the same radio (BC-780). Experiment and see what works good.

 

[K4NRG]

I've had good luck with a 1/4 wave VHF Hi band antenna as well. I now have an Austin Spectra but have not seen a notable difference especially on UHF air.

 

[ka3jjz]

Not surprising; the Spectra isn't tuned for that band. It's set up for the 440 mhz and up as 'UHF' (it hears 70cm ham quite well - I've even heard tropo on it). That's where it really does well. Neither is it tuned for the 138-144 mhz area.

A shame, but certainly not surprising. 73s Mike

 

[VernM]

Some guys really delight in over-engineering and throwing money at things. Like it says up there in the second message, nine inches or so of wire and a good receiver and that's about the best you can do at any price. Look at the aircraft. Their antennas aren't to fancy, mostly quarter waves in a fairing to reduce drag and support the stub at high speeds. It doesn't take much, really, when you have altitude working for you.