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Air Traffic Monitoring => Feed Setup Pictures => Topic started by: Squawk 7700 on February 02, 2014, 02:26:39 AM

Title: NorCal Antenna Photos
Post by: Squawk 7700 on February 02, 2014, 02:26:39 AM
Here are some photos of my recent antenna upgrade. The "upgrade" was mainly to raise the beam up to about 15 feet from it's previous 6 feet off the roof, and to guy wire the mast for stability in high winds.

The base can pivot so I can easily lower the whole antenna if needed by unlatching one of the guy lines. The Phillystran guy lines are non-conductive, and the mast is made of fiberglass.

The 6 element Aircraft Band Yagi was built by Directive Systems. It's advertised gain is 9.2 dBd with a frequency range of 118-128 Mhz
My Omni Antenna is a "SlimJim" antenna made out of 450 ohm ladderline tucked inside the uppermost part of the mast. Using LMR-400 cable on both antennas.

Some notes: Speaker Stands make great antenna stands as well. Used thread lock compound on the guy line hardware. Hose Clamps work great at tightening the fiberglass tubes together, but you need to hacksaw a slit on the end so it can squeeze in on the inner tube. But just in case, I used a screw as well.  The original red endcaps on the beam faded over time and eventually cracked so I replaced them with newer black endcaps. The mast and Phillystran were purchased at DX Engineering.


Title: Re: NorCal Antenna Photos
Post by: RonR on February 02, 2014, 09:05:34 AM

I'll bet you can hear JFK with that antenna  :-D

Just curious, any reason why you didn't mount that beam closer to the top of the mast above the guy wires?  Or do you have other plans for that part of the mast?

Title: Re: NorCal Antenna Photos
Post by: Squawk 7700 on February 02, 2014, 12:56:21 PM
Hi Ron,
Yes, that was actually the original idea with the beam over the guy ring but when I tried that the coax had to go over the hump of the guy ring as it travelled down the mast. No big deal I just wanted a finished appearance. If I used metal guy lines I would have stayed above the guy lines. I could place a smaller beam say for MilComms above? The upper 5 feet of the mast is where the VHF omni is, although looking at the photo it looks like the mast is bare. Nice thing with the tilt over is I can try out new things easier.
Good thing I found the caulking gun yesterday and patched the roof. It raining here today.
Title: Re: NorCal Antenna Photos
Post by: InterpreDemon on February 02, 2014, 03:38:28 PM
Ken, FWIW, even if that mast was not metallic (though I suspect it is) the coax feed(s) are, and would be distorting your pattern pretty severely to the point where you are not getting the 9-10db you should be getting out of that 6-el, in fact I'd be surprised if you were getting half, which may be why it took you so long to align it... the peaks and nulls are not where you think they should be. It also makes it a bitch to tune, and your bandwidth gets pretty narrow with six elements to begin with. Even if your mast was PVC, the coax going up to the omni would essentially make it a metal mast, so there is no escape in the present configuration.

If it were me, I would twist that beam horizontal, which even though you ordinarily would lose 3db for the vertically polarized ground antennas (over a properly mounted vertical) would make little difference with the aircraft, would reduce AM impulse interference, restore the gain and pattern, thus probably working better overall. I would also want to get it above the metallic guys, which are more problematic when they are angled closer to the plane of the elements and passing nearer to the high impedance ends than, say, terminating at a guy ring right beneath the boom of a horizontal array. If you can't get the antenna above the guys, you could at least break up the guys at their upper ends with multiple insulators, say one every 18" for any guy within four feet of an array element. Even simpler, just replace the cable with 1/4" polyester line from the marine store... stretch and sunlight resistant, absorbes shock better than cable and for your short mast will work just fine.

Otherwise, the only way to get full design performance out of a vertical yagi is to side-mount it off the mast by at least a quarter wave (2 ft), or end-mount it from the rear, though yours is getting pretty long for an end mount and "weather vaning" becomes an issue unless it is heavy-duty commercial stuff (2 and 4 el examples attached).

There again, if it's good enough as is... it's good enough. Sometimes "better" is the enemy of "good enough".
Title: Re: NorCal Antenna Photos
Post by: Squawk 7700 on February 02, 2014, 06:33:09 PM
Thanks for the suggestions! I will certainly try rotating it to the horizontal plane and move it up above the guy line and see the signal effects. Looks like my antenna project continues to evolve.  :-D

That's a nice setup you have there.