915 MHz Coaxial Collinear conversion to 1090 MHz project

#1
When I first considered the idea of converting this old ATV Coaxial Collinear to 1090 MHz,
and saw how long the FG Radome was, I wondered if it was a bit too long..

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It seems to have solid core RG-58 type coax inside, and it might not be too hard to Hack the existing elements
to the correct size for 1090.. Maybe just cut out every-other joint, trim, strip & re-solder them..
Those 915 MHz Short and Open stubs shouldn't be too hard to replace. If I wanted to keep the same design.

I'm thinking this project might be a good use of the antenna analyzer project I'm working on..
Currently waiting for hardware from China. http://forum.planefinder.net/threads/how-to-tune-a-filter.135/

If I can get the analyzer working at 915 Mhz, and measure this antenna 'as is', and then with the 1st element only,
maybe I can make the first Mods, with the analyzer running, using the readings as a guide to get the right length of first elements, before making the mass hack of the rest..

So, the question remains, how tall should I make this sucker?
If it was 1 meter tall, and I added another meter, it would add 3 dB of gain..
But, 3dB is doubling the power, and might add an extra 30 miles to a 1 meter tall coco..


Edit: July 8, 2014
There might be a very easy way to get this done, using a simple calculation.
If I can get an accurate measurement of the 915 antennas resonate peak,
(should be between 910 & 930 Mhz) then I can calculate the amount to trim each element.
I think it will be around 16%.. This could be easier than it looks..



Your comments please:

Cheers,
Rich
 
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#2
Today, I was able to measure the antenna's resonate frequency. The working freq is 917.5 MHz.
I think the coax segments need to be shortened about 15.85%

So, if a segment is 107 mm, it will need to be cut down to 90 mm.. 17 mm in the trashcan..
 
#3
This might not be all that easy. I removed the two coax stubs (see Short & Open in pic below)
and re-checked the return loss at 917.5 MHz and it was bad, like 6dB bad..

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Not sure what to make of this antenna now. I'm starting to think we were scammed.

About 15 years ago, when I was making L-band antennas for 'the Man', and old RF engineer told me about some
of the tricks that lazy antenna makers have used to get their antennas passing grades.

They would add attenuation during calibration of test equipment and remove it before testing the antenna.
That makes the antenna look better than it really is. Wow, this thing has a ton of gain!

The other trick is to tune the 50 ohm feed, tweak the matching section, to make it a good match.
So, the poorly designed antenna has a very good SWR, which makes it seem to be working fine.

It just doesn't have much range, because the feed point matching components were radiating all the power,
off in odd directions, instead of driving the antenna's elements correctly for max gain.

In one case, a 'worker' installed a hidden 50 ohm resistor to give the antenna a perfect match..

It's possible to replace the driven element on a 220 MHz yagi antenna with a driven element made for a 440 Mhz yagi antenna.
You could tune and tweak that 440 driven element, until you had an excellent SWR.. And, it would work to some degree..
But the directional gain would be awful.. :eek:



While removing the two stubs, I noticed something strange. The center conductor of the feed (brass coax),
was not connected to the braid of the first element.
Instead, it was connected to the center conductor of the little Short stub.

1. So, the RF comes in on the bass coax center-conductor,
2. feeds into the Short stub center conductor,
3. hits the twisted braid tail connection with the stub center-conductor,
4. and comes backwards down on the stub's braid,
5. to a solder connection with the braid of the 1st coaxial element.

The center-conductor of the Open stub was also connected to the braid at that point.
It's braid was soldered to the brass.

That round-about feeding of the input doesn't look at all like a standard coco design (see pic below).

I'm not sure, but I have a feeling the Short stub (and maybe the Open stub too) comprised a little
miniature resonate/mini-antenna, that will accept 917 Mhz power from a transmitter,
and radiate most of the power right there, at the base of the antenna.
With a small portion of the power radiated from the many segments above the 'tuning' section.
Kinda like what Hams call a Long-Wire tuner rig.. AKA random wire antenna or end fed longwire antenna.

The antenna was built, had a horrible SWR, so it was 'adjusted' to make it appear to be working..

Lastly, this antenna has 18 sections of coax! The top section is not terminated, just open.

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