5/8, 1/2 or 1/4? (REALLY controversal topic)

[VK4WDM]

I want to build a receiving-only vertical for the 122Mhz (airband). The question is: which would be best, 5/8, 1/2 or 1/4 wave-length?

Please explain your reasoning so others can learn.

I put his here rather than in the "aircraft enhancement" segment because the 2m ops will have build heaps of these.

73

Wayne VK4WDM

[VK5ZD]

Hi

Given that you want to receive signals from sources that may be at a significant elevation angle (i.e. aircraft), I'd suggest the 1/4 wave.

73
Iain

[VK5ACY]

G'day Wayne,

The 'J-Pole' antenna (aka 'Slim Jim' etc.) is easy to build and to 'get going'.

I built such an antenna for the Aircraft Band many years ago and it still works fine.

The J-Pole is a vertical - perfect for Airband use - and can be scaled from a 146MHz design or you
can use one of many 'calculators' to be found on the internet..e.g. :-

http://www.hamuniverse.com/jpole.html

http://www.kb3kai.com/j-pole-calculator.php

http://www.aralb.org/Commitees/Technica ... ntenna.htm

http://www.m0ukd.com/Calculators/Slim_Jim/index.php

.....there will be many more !

Easy to build out of 'Junk Box bits', vertically polarised, no elaborate groundplane requirements....

Have fun !
Bill VK5ACY
PF95sb

[VK4GHZ]

...or something else!

I have a Diamond D-130 Discone.
I can hear aircraft sitting on the apron at BNE airport talking to the tower.
This is 30km away, and NOT line of sight.
The discone is only at a modest 20' above ground.
My observation is: it works well!

This feeds my IC-R1500 receiver, so the discone becomes useful for general purpose monitoring on 6m, 2m and 70cm.
A lot of bang for the buck.

[VK4WDM]

Hi Adam

I Agree about the D-130. That's what I am using here. It also does a reasonable job on 1090MHZ for Plane Plotter.

The new antenna is for someone else who has just got interested in listening to the airband. He has done some reading and thought that he would need a 5/8 whip, somebody told him a 1/2, and I suggested a 1/4 GP.

He only lives a few k's away from the airport so gain is not an issue.

73

Wayne VK4WDM

[VK4WDM]

Given that you want to receive signals from sources that may be at a significant elevation angle (i.e. aircraft), I'd suggest the 1/4 wave.

Hi Iain

Can you give a short explanation why that suggestion? What advantage does a 1/4 wave over the 5/8. The latter is longer, surely it should work better?

73

Wayne VK4WDM

[VK5ZD]

Given that you want to receive signals from sources that may be at a significant elevation angle (i.e. aircraft), I'd suggest the 1/4 wave.

Can you give a short explanation why that suggestion? What advantage does a 1/4 wave over the 5/8. The latter is longer, surely it should work better?

The longer antennas (1/2 & 5/8 wave) provide gain by lowering the radiation angle. i.e. they work best with signals being sent or received from the horizon. A 1/4 vertical is closer to a semi-spherical radiation pattern (though there is still a null directly overhead). If you just want to listen to signal from an airport then the 5/8 wave would be the best choice. If you want to receive signal from flying aircraft, particularly those nearer your QTH where the elevation angle is higher, then the 1/4 wave may be better option.

73
Iain

[VK3HZ]

If the aircraft is at a high angle, then it'll be fairly close and so strong that you'll receive it on just about anything. It's when the aircraft is a long way off - and thus close to the horizon - that you'd want the most gain. The longer antenna should do better.

I haven't tried VHF, but on 1090 MHz ADS-B, at home I use an 8-element vertical collinear to give max gain on the horizon. In the field, I use a 17-element yagi pointing at the horizon to get best range. Even with the narrow beamwidth of the yagi, I see aircraft all the way in, overhead and even behind if they are fairly close.

Regards,
Dave.

[VK4WDM]

I made a 1/4 wave GP from a PL259 socket and a bit of brazing rod. Plain vanilla, but it works well

73

Wayne VK4WDM