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HomerBB
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Post subject: Pulsar Miniature 2 Element Beam?
 Posted: Apr 28, 2012 - 02:18 PM
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DX Alpha
Joined: Mar 18, 2010
Posts: 687
Location: NW Arkansas USA
Status: Offline
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This antenna photo showed up on a couple of sites I am a member of.
The discussion centered around two things- 1. Have you seen this?, 2. Does it work as advertised?
It is advertised to be a portable 11 meter beam.
I entered the first discussion because one of the forum members emailed me asking whether I would be willing to make one and test it to answer the questions of its true nature.
I told the second forum that I had said I would do this when I got some time.
I'm inviting the DXRW in on the fun.
So far:
quotes from other places:
"I used copper tubing to put one together based on the posted specs. Until I get it out in the open air and coax on it I can't be sure, but suspended in the garage with a short jumper it does not approach a readable SWR on any frequency. I can not tell by any of the drawings whether there is more to the antenna than as described in previous posts. More to come with photos."
"Ok, I put it on a step ladder and hooked up the analyzer. Details later.
I also hooked up a small AM radio and listened, and got a radio check from a truck driver on ch 19: video to come of that.
It has an atrocious SWR, and the X=0 resonance is elsewhere.
Photos and more comments to come. I am due at work in 10 minutes."
"I want to add that I believe this antenna may have potential in a limited way.
I have used copper tubing because I wanted to reuse it for another project when done with this.
I have most of the aluminum strap to go exactly to the materials specs and will get the remaining piece to do it up and see. . ."
"Two quick photos and a video link to the first on-air test.
Where it shows X=0 resonance:
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"Where the supposed center frequency is supposed to be - not quite there in this model:
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"And a video link of the first test:"
In Touch - 2el Miniature Pulsar Antenna Homebrew |
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WR954
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Post subject: RE: Pulsar Miniature 2 Element Beam?
Posted: Apr 28, 2012 - 03:58 PM
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Vice-President
Joined: Jul 28, 2008
Posts: 535
Location: Grass Valley, California
Status: Offline
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| Well, I can see you playing with it some more, but I do not think you will see the great numbers there that they claim it is. Just too much reactance/capacitance issues. And the R factor at resonance, WOW, lower the a stick omni would theoretically be which is around 30 ohms, hence the biggest reason for radials as the starting was to raise the feedpoints resistance closer to 50 ohm by using aroun a 45 degree sloping ground radial while trying to maintain the resoanace X factor. I would be seriously wondering if some measuremen t or another is incorrect and it is not making a smooth RF path between director and reflector. That is my suspicion. Something to do with the spacing. |
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HomerBB
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Post subject: RE: Pulsar Miniature 2 Element Beam?
Posted: Apr 28, 2012 - 07:35 PM
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DX Alpha
Joined: Mar 18, 2010
Posts: 687
Location: NW Arkansas USA
Status: Offline
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Those are some reasonable thoughts, and worth considering.
I probably won't be spending a lot of time on this. I was asked to do an honest. objective report on it.
I said I would. To do that I will have to reconstruct it to specs and try it.
I will do that, put a feed line on it that I know is correct from a known good dummy load. Raise it to practical portable heights - 12' to 20', and then tell them what it does with my evidence. Personally, I was surprised at the level of receive and report I got back from the truck driver on it in the video. |
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HomerBB
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Post subject:
Posted: Apr 30, 2012 - 03:27 AM
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DX Alpha
Joined: Mar 18, 2010
Posts: 687
Location: NW Arkansas USA
Status: Offline
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Ok. The latest, and despite my previous commitment to redo the thing to exact materials specs, perhaps my last.
Redoing it to the exact specs was an extension of my belief that the antenna might have some possibilities as a portable system.
That was based on some encouraging results and the video with a decent radio check I recorded.
Of course, on some of the forums I said I did not believe it was a beam, and still don't. It is a dipole, off center fed. The feed system settles that - braid to one half the antenna, center conductor to the other. Simple.
Today I decided to test a thought that occurred to me based on experience with other types of shortened antennas, NGPs (no ground plane) in particular. Those antennas have a single thing in common - they depend on the coax to be part of the radiating system. To test this idea I mounted the antenna just as before, but this time I inserted a coax choke one foot from the connection point of the feed line to the antenna to defeat any potential CMC, and thus prevent the coax from being a radiating element.
Just as I suspected, anything remotely resembling a match, or resonance across multiple bands wasn't happening for me. It is my opinion that this antenna depends on the coax to be part of the radiator.
Here is my personal conclusion:
Can it be used as a portable antenna on 11 meters?
Yes, but you must be careful of the coax length in order to get a match. Is there a special length to be used? I doubt it. Just so there is enough to get the job done should do. Too short a coax will not provide enough length to radiate at the 11 meters band. I suspect that conductive surfaces near it will likely affect it.
Is the antenna a beam? No. It's a dipole - a split element, off center fed dipole.
Should one buy one? If they wish. It isn't useless, and could be fun. It certainly doesn't use too much space as many 11 meter antennas tend to do.
Would I buy one? Of course not, I already have one.  |
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WR954
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Post subject:
Posted: May 01, 2012 - 08:58 PM
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Vice-President
Joined: Jul 28, 2008
Posts: 535
Location: Grass Valley, California
Status: Offline
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