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#1
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Voltage feeding a VHF yagi
Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub like a J-pole? A quick EZNEC run seems to indicate that it's not a bad idea. So it seems like it might work OK... but I wonder if anyone has actually done it. Would you expect a coaxial stub to work better than a parallel wire stub in terms of preserving the pattern of the yagi? Any commercial products doing this? 73 Dan N3OX www.n3ox.net |
#3
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Voltage feeding a VHF yagi
wrote: Anyone ever get around the VHF FM vertical yagi feedline routing problem by voltage feeding the bottom of the driven element with a stub like a J-pole? A quick EZNEC run seems to indicate that it's not a bad idea. So it seems like it might work OK... but I wonder if anyone has actually done it. Would you expect a coaxial stub to work better than a parallel wire stub in terms of preserving the pattern of the yagi? Any commercial products doing this? 73 Dan N3OX www.n3ox.net I guess I'm missing it, but what problem is there running the feedline to the normal center fed yagi? No matter if against a mast, or mounted on a mast with the center of the boom attached to the mast, I don't see a problem. I do it all the time. No matter what polarization, I tape the coax to the mast, and then tape it to the boom to the feedpoint. I'd much rather run the yagi in the normal manner. Trying to voltage feed one sounds like a common mode nightmare , unless drastic precautions were taken. And I probably don't have to mention how radiation from a feedline can skew a antenna pattern real nicely... MK |
#4
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Voltage feeding a VHF yagi
I guess I'm missing it, but what problem is there running the feedline to the normal center fed yagi? No matter if against a mast, or mounted on a mast with the center of the boom attached to the mast, I don't see a problem. I do it all the time. No matter what polarization, I tape the coax to the mast, and then tape it to the boom to the feedpoint. I'd much rather run the yagi in the normal manner. This is a problem that is particular to vertically polarized yagis that are mounted with a pole going between the elements. The pole has to come up parallel to the elements to the boom. Some people overcome this by using non-conductive pole, BUT what do you do with the feeder, if you tape it to the pole you might just as well have used a metal pole!!! However, as I said in my previous post, my simulations seem to show that with careful positioning the effects can to quite small 73 Jeff |
#5
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Voltage feeding a VHF yagi
Jeff wrote:
This is a problem that is particular to vertically polarized yagis that are mounted with a pole going between the elements. The pole has to come up parallel to the elements to the boom. Some people overcome this by using non-conductive pole, BUT what do you do with the feeder, if you tape it to the pole you might just as well have used a metal pole!!! However, as I said in my previous post, my simulations seem to show that with careful positioning the effects can to quite small This problem has been investigated very carefully by moonbouncers who want to use selectable horizontal/vertical polarization. The best solution from the electrical viewpoint is to mount the yagi on a fibreglass mast and route the feedlines out the rear, but this is not mechanically practical for very large steerable arrays. Simulations have found that metal masts in the plane of the elements can be used, and feedlines can follow the same route, if - BUT ONLY IF - the yagi is suitable and it's done at the right place along the boom. Under these conditions, the penalty (in terms of forward gain) can be less than 1dB. However, only certain yagi designs will be suitable, because the support point must also be close to the mechanical centre of gravity. These electrical and mechanical requirements may conflict, so I strongly agree with Jeff that the effect of the mast and feedline will always need to be simulated and computer-optimized. There is no way to guess at this one. Coming back to the original question about a J-pole feed for a vertically polarized yagi, this technique was used in the 1950s-60s by the British company J-Beam for TV antennas around 50MHz. The driven elements that I remember were not actually J-shaped, but used a coaxial decoupling sleeve that also served as a mast. The bottom end of the sleeve was typically clamped to a chimney. It worked... but in those days, nobody really understood how well or poorly it worked. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#6
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Voltage feeding a VHF yagi
Jeff wrote: I guess I'm missing it, but what problem is there running the feedline to the normal center fed yagi? No matter if against a mast, or mounted on a mast with the center of the boom attached to the mast, I don't see a problem. I do it all the time. No matter what polarization, I tape the coax to the mast, and then tape it to the boom to the feedpoint. I'd much rather run the yagi in the normal manner. This is a problem that is particular to vertically polarized yagis that are mounted with a pole going between the elements. The pole has to come up parallel to the elements to the boom. Some people overcome this by using non-conductive pole, BUT what do you do with the feeder, if you tape it to the pole you might just as well have used a metal pole!!! However, as I said in my previous post, my simulations seem to show that with careful positioning the effects can to quite small 73 Jeff I think normally the effect is fairly small. But I suppose it can vary per the position, and mast length, etc.. I guess it's all how picky you are about the gain/pattern. For moonbounce, etc, it might be worth worrying about. But for just a general purpose FM beam, I probably wouldn't worry too much. Unless the mast was a certain length to become resonant, it should pretty much be ignored by the antenna for the most part. Seems to me, right in the middle of the two midpoint elements would be about best, but I've never modeled other positions. I have seen quite a few people run vertical yagis with the midpoint mast mount with no serious problems. I think any small loss of performance would be less than trying to voltage feed thats for sure. You'll see lots of antenna ad's with pictures of vertical beams mounted on masts in that manner, so it shouldn't be too bad. I agree. There should be little difference in using a metal mast, or plastic, if the feedline shield is in place on both. I'd just as soon use the metal mast I think. It would look fatter with the extra metal mast, but I don't know if that would be a plus or a minus... MK |
#7
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Voltage feeding a VHF yagi
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#8
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Voltage feeding a VHF yagi
I forgot to say you could offset the coax with a crossarm out from an
insulated mast, not offset the Yagi. With a metal mast you would offset the Yagi. |
#9
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Voltage feeding a VHF yagi
I havent but Mr Cebik talks about it on his website;
http://www.cebik.com/vhf/jp4.html Cheers Bob VK2YQA wrote: Anyone ever get around the VHF FM vertical yagi feedline routing problem by voltage feeding the bottom of the driven element with a stub like a J-pole? A quick EZNEC run seems to indicate that it's not a bad idea. |
#10
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Voltage feeding a VHF yagi
Bob Bob wrote:
I havent but Mr Cebik talks about it on his website; http://www.cebik.com/vhf/jp4.html Cheers Bob VK2YQA wrote: Anyone ever get around the VHF FM vertical yagi feedline routing problem by voltage feeding the bottom of the driven element with a stub like a J-pole? A quick EZNEC run seems to indicate that it's not a bad idea. The problem with many models is people just stick a source at the feedpoint and assume it tells them something about how the antenna works in the real world. A model has a PERFECT source that is like using a perfect balun and a transparent feedline. I can build a very feedline critical antenna with horrible balance issues, one that will never work in the real world, and make it behave quite well in the model. I would never use a J-pole feed on a Yagi, except perhaps in a model. 73 Tom |
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