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More loop comments
I may have given the impression that I think that loops have no place
in a radio system. I am also a ham, extra class, and for the last 18 months I have been somewhat active using NVIS in a mobile setup. When you are using a 1991 Honda roof as your ground plane, a loop is the only thing that works. My negative comments about loops should be for broad band, untuned loops. My SWL mainly is utility and I often have 2 receivers tuned to very different frequencies. For my SWL activities an antenna that must be tuned is not very useful. A friend pointed out that I had tared all loops with the same brush. I will stand by the observation that in all of my tests, small, fractional wavelength loops have no advantage over, and have proven very inferrior to the Lankford active dipole. Even a north country configured in a dipole array worked better then the Wellbrook. I will be the first to admit that I have not experimented with larger loops for broad band SWL reception. If I every get the time it would be interesting to use a Dallas "ultra linear" amplifer with a 50' square horizontal loop. For the time being I will concentrate my energy on standard "long" wire antennas with a set of 2 vertical Lankford active dipoles couple to a phaser with an additional and rotatable horizontal active dipole. That should cover my foreseeable needs. A Langford "ultra linear amp" makes a very good buffer for ampifing a signal prior to power division. In fact it works better, as in much higher IP2, IP3 and simple harmonic distortion then the 40 year old Nuvistor based unit I had been using for the last year or so. FWIW we got the desk in place, stained and sealed between showers today. And I was able to burry the copper tubing that will cary the coax away from our home. You can't have too good a ground. The entry bulkhead is ready to mount, and I am only a week behind on my planed timeline. Terry |
#2
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More loop comments
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#3
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More loop comments
Telamon wrote: What equipment are you using to make the IP2 and IP3 measurements? Can you share the comparison data you took on the units? -- Telamon Ventura, California I didn't do the tests, a graduate (PHD) engineering student I know ran the tests for me. I will ask for the exact equipment used.As for the exact data that will take a while, my "ham shack", communicaitons room radio room is packed in boxes for the renovation. I know the people at a local EMI certification lab, full screen room etc, and I am hoping to run some tests on all of my active antenna, the "soon" to be completed WL1030 and the 1530. Terry |
#4
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More loop comments
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#5
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More loop comments
Telamon wrote:
Well for the measurements to be meaningful the test conditions must be known. You must (best) know the tone frequencies and input levels along with the output tone levels and the levels of all the intermodulation products you want to report on. IP2 and IP3 are specifications that are higher the better. If I'm not confusing you with another thread I think you mean a comparison to the ALA1530 you you writ "1530". The ALA1530 has very good specifications and to quote this page: http://www.wellbrook.uk.com/ALA1530.html "The Broadband Loop has been specifically designed to reduce intermodulation products to a minimum. The second order and the third order intercept points are typically +70dBm (IP2) and +40dBm (IP3) respectively. Thus the level of the intermodulation products are generally below the atmospheric and man made noise." These intermodulation numbers are huge and I don't see the point in working to get higher numbers than these. I could say that numbers higher than these are not important unless you sit by some transmitter in the passband of the antenna/amplifier. I am I off base? -- Telamon Ventura, California One reason I want to get the Wellbrook teste, is to verify that it is fact operating correctly. The guy I traded if from says the performance now is like when it was new. His dissatifaction lead, in part, to his moving on from SWL. Of course even if everything had worked perfectly he would have grown bored in a few months to a couple of years. My comments regarding Dalla's "ultra linear amplifier" had to do with its use as a large loop, non fractional wavelength,amplifier, and as a buffer to make up the losses in a 4 way power divider setup. The ancient GE multiset coupler uses Nuvistors and Dallas' amplifier has much better IP2/IP3/THD then that unit. Of course it still has the original tubes and I am running plates at about 5 volts lower then the orignal design. Dallas' amplifier being balanced and having a better handle on negative feedback should be expected to have lower distortion then a Nuvistor triode amplifier. I completed the WL1030 and, at my home, it behaves very similary to the Wellbrook ALA. Slighlty better 3rd harmonic distortion from the local 770KHz MW station a few miles away, but still not as good as the stock North Country active antenna, a Datong AD370 or stock AMRAD antennas as published in QST. I used a Kiwa highpass, BCB reject filter, on the output of all three antennas. Compared to Dallas' active dipole all the others are kind of sad. Receivers used: R2000 R8B R392 Hopefully this evening I will get a chance to run the WL1040 and a just completed "improved" version of Dallas' active dipole to Jeff's condo for a real world, nightmare test. All of the antennas except for the wellbrook can be easily tested as one has access to the amplifiier input. Sadly the Wellbrook is in ?Epoxy making testing and/or repair difficult if not impossible. This thread was meant to define what I consider to be a "good" useage of small loops. That is for mobile or other space restricted spaces where a tuned small loop can give amazing results. I have been underwealmed by broad band, untuned loops. I am not saying they don't work, I just haven't exeperienced a broadband loop that works as good as a simple active antenna, much less a proper active antenna. Using the small loop on 20 and 40 meters and NVIS I have talked/CWed mobile from Knoxville TN to Lexington KY. That is a successfull aplication of a loop antenna. Terry |
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