Need Help Designing Mobile Antenna

[K6IRM]

Hey All!

I need some help making a mobile (NMO mount) 5/8 wave antenna for airband (126.5)

So far I've come up with two possible ways to accomplish this, but I wanted to see if any of you have a better idea.

Here's what I got:

Option 1: Use a 2M mobile NMO antenna, replace the whip with something longer and cut it down to tune.

Option 2: Use a 6M mobile NMO antenna, cut it down to tune.

Option 3: You're doing it wrong. I have something better. Lemme tell ya....

 

[KevinC]

I moved your question here for better visibility.

 

[prcguy]

Starting with a 2m version will be easier but the whip length may be hard to find. Do you want to get te entier VHF air band or just 126.5? A 5/8 will be fairly narrow band and will not work well except for a few MHz.
prcguy

 

[cmjonesinc]

DPD Productions - VHF Air/Aviation Band, FBO Base, PCL, AWOS, ACARS & ADS-B (SBS / RadarBox) Antennas for Commercial & Hobbyist Applications not the cheapest choice, but a choice all the same.

 

[AI7PM]

Why 5/8?

 

[gewecke]

Cut a quarter wave Nmo at 18" and you'll be good to go. . 73, n9zas

 

[prcguy]

Since this is for VHF airband, I think something around 25" is more appropriate.
prcguy

Cut a quarter wave Nmo at 18" and you'll be good to go. . 73, n9zas

 

[N5TWB]

Why 5/8?

Exactly my question -- for receiving purposes, you likely won't notice any performance difference by using a 1/4 wave, an easily available purchase.

 

[N3JI]

For listening to the plane side of the conversations, a 5/8ths probably ISN'T what you want -- if focuses more towards the horizon. A 1/4 wave will have more coverage towards the sky.

Like this:

 

[gewecke]

Since this is for VHF airband, I think something around 25" is more appropriate.
prcguy

. Strangely enough, mine seems to perform better on aircraft at 18" :wink: 73, n9zas

 

[K6IRM]

The antenna will be used for transmitting. Its for an airport fire truck.

Needs to be 5/8 because the current 1/4 just isn't enough combined with the 2 watt transmitter.

I understand it will reduce bandwidth (only about 4mhz up and down will be used)

My biggest issue is matching the antenna correctly, which I have absolutely no clue how to accomplish.

Thanks!

 

[prcguy]

You want low angle for aircraft because aircraft up at the high angles are fairly close to you and usually direct line of sight. You can probably pick them up with an exposed PL-259 connector and no antenna. You want the low angle for better performance when aircraft get below line of sight behind buildings, trees, etc.

Calculate the distance to an aircraft at say 35,000ft and a 30 or 45deg look angle, they are not that far away where a 5/8 would have any problems with lobes or nulls.
prcguy

For listening to the plane side of the conversations, a 5/8ths probably ISN'T what you want -- if focuses more towards the horizon. A 1/4 wave will have more coverage towards the sky.

Like this:

 

[cmdrwill]

Here is what I use for 'airband':

Comtelco A1511AAC covers the full band and does not flop around like the skinny whips.

"Needs to be 5/8 because the current 1/4 just isn't enough combined with the 2 watt transmitter."
Something must be wrong with the antenna or mount/coax.

 

[N3JI]

The antenna will be used for transmitting. Its for an airport fire truck.

Needs to be 5/8 because the current 1/4 just isn't enough combined with the 2 watt transmitter.

I understand it will reduce bandwidth (only about 4mhz up and down will be used)

My biggest issue is matching the antenna correctly, which I have absolutely no clue how to accomplish.

Thanks!

This info would be handy from the start :wink:. With a 2W transmitter limitation, I'm not sure 3dB gain will be a lot of help. But to make it work, all you need is the 5/8ths whip and a matching coil at the base. Should be easy enough to calculate the coil you'll need.

EDIT: Here's an innovative build that you might be able to adapt to the aircraft band:
http://www.arrl.org/files/file/Technology/tis/info/pdf/8009022.pdf

 

[prcguy]

If you want any advantage of a 5/8 antenna over a 1/4 wave whip you don't want to shorten the 5/8 much from its normal full length. The matching coils used in most commercial 5/8 mobile whips only shorten the antenna a small amount so I would base your antenna on the same design.

With a little back of a napkin calculation I think you can do it by getting a Larson or Maxrad or similar VHF 144-174MHz 5/8 mobile antenna and replace the stock 49" whip with one about 53" long to resonate at 126.5MHz. Ebay has 62.5" long whips for under $16 including shipping that will probably adapt to the coil of the antennas mentioned and should tune up ok.

Worst case you might have to enlarge the whip hole in the coil to accept a thicker whip and here is a link to the longer whip: Wilson 880 900901 W1000 W5000 Series 62 5" CB Antenna Whip | eBay

It sounds like this antenna is for a fairly important system and you probably don't want to cobble something together that could fail and put people or property in jeopardy. I would recommend using off the shelf antennas, mounts or parts and do a thorough mechanical test of the finished product to make sure it will stand up to your worst expected abuse before putting into service.
prcguy

The antenna will be used for transmitting. Its for an airport fire truck.

Needs to be 5/8 because the current 1/4 just isn't enough combined with the 2 watt transmitter.

I understand it will reduce bandwidth (only about 4mhz up and down will be used)

My biggest issue is matching the antenna correctly, which I have absolutely no clue how to accomplish.

Thanks!

 

[jeatock]

A commercial 5/8 with a longer whip should work. Actual length is a matter of theoretical physics.

Ground to air is more a function of the aircraft's height, and a gain antenna works by stealing high angle signal and redirecting it towards the horizon. Probably not your best option.

That said, since this is a potentially life and death installation I would hire the services of a local radio shop with an Anritsu Site Master to come out and fine tune it. A 10-minute job in reality, but the $100 service call offsets the potential liability.

In agreement with previous posts: if you can't hear on a 1/4 wave there may be something else wrong. As far as transmit range, 2 watts should be plenty to cover an airport. It sounds like there may be another issue. See above paragraph.

If 2 watts isn't enough horsepower to cover the airport Ground to Ground because of obstructions or high ambient noise floor, you may be better served by updating the radio. GSA pricing on a new A120 9 watt mobile radio should be under $900.