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Author Topic: Vertical yagi on a metal tower - interaction?  (Read 4197 times)
GM7CXM
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« on: April 21, 2013, 10:22:37 AM »

I am one of those lucky guys who is able to operate mobile from beside the sea. In an effort to improve signals even more, I have done a few tests using portable vertical yagis, using quarter wave elements and radials, which have turned out more or less fine. However those antennas by definition are not rotatable. One option I am considering is a 2 element vertical yagi on a small mast, however I am unsure whether the metal mast would affect the pattern/performance of the antenna. I suppose that if the mast is a non-resonant length then it would not affect things much but, does anyone have practical experience of this? I am thinking of monoband antennas from 20 to 10 metres. Any comments welcome!

Thanks and 73 de Duncan EA5ON / GM7CXM
www.ea5on.com
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NH7O
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« Reply #1 on: April 21, 2013, 10:49:37 AM »

I think the answer will have to be one those "it depends" types. Modeling is always a good idea. One thing to try is to check the antenna with and without the presence of the mast, and look for SWR change. If there is appreciable interaction, then you could try detuning the mast. That involves adding a shunt wire loop and a tuning capacitor. See w8ji.com for detailed info on that.

Otherwise, if there was a way to add insulated couplers to the mast, then interaction would be nil. This is similar to the old HyGain quad that used metal spreaders broken up with insulators.

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AH6RR
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« Reply #2 on: April 21, 2013, 11:29:11 AM »

You could always get a School bus and mount the antennas on the roof then you can point it anywhere you want. One of my Elmer's did just that he had it set up for 20-10 meters and it had 3 elements, he would park it within 10 to 20 feet of the water and he would use a chain attached to the frame that he would toss into the saltwater for a ground, it worked like gangbusters.  I sat and watched him work the world with it. When I got my Tech+ license I had a Dodge Van (Full size) and I did a 2 element for 10 meters on the roof using 102" whips and did the same thing and it was fantastic with just 100W. Of course this is when I lived in Texas where you could drive right up to the water it is a lot harder to do that here in Hawaii.
 But as Al pointed out it depends you could always make a tripod and use a Pole vaulters pole (fiberglass) for the boom to hold the elements and that would pivot on the tripod and have elevated radials but you would be limited to the higher bands to make it manageable and it could be a pain to turn since you would have to move the radials each time you turn it. 

Good Luck it sounds like something worth trying though.

Roland AH6RR
 
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GM7CXM
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« Reply #3 on: April 21, 2013, 11:56:41 AM »

Al: Thanks for your comments. I'm not much good at modelling, but have a friend who can help. Ideally the mast would be non-conductive but I don't think that's very realistic. And not sure that inserting insulators is too easy either. So maybe modelling is a good idea, yes.

Roland: I've done the two element for 10 metres on top of my small car and it works OK, yes. Getting a school bus is not an option, but you have just given me an idea. I was toying with the thought of a trailer tower such as the ones made by Tennamast (a UK company) and putting a beam and rotator on it. But, I could also set up a yagi on a trailer and drive it round to change direction. Now let me go and think about THAT for a while!

Thanks and 73
Duncan
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NH7O
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« Reply #4 on: April 21, 2013, 06:48:57 PM »

I see that I did work you, remember the qso. You are already getting out well. Here in Hawaii, Roland and I and everybody else get to listen to the guys right on the north shore O'ahu beach. They use a vertical yagi on 20m and get out way too well. No real ground is required.

Another thought is the so called "garden yagi" which uses vertical 1/4 wave elements as radiators above the boom, and horizontal 1/2 wave elements as ground planes. That means a bit of additional tubing and overall size compared to just two vertical elements. Such a design would decouple the radiators from the tower effectively, so anything metallic would not be a concern. get it near salt water and it will do fine.
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RFRY
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« Reply #5 on: April 22, 2013, 05:35:46 AM »

... I am considering is a 2 element vertical yagi on a small mast, however I am unsure whether the metal mast would affect the pattern/performance of the antenna. I suppose that if the mast is a non-resonant length then it would not affect things much ...

Below is a link to a NEC4.2 study about this.

For those conditions it does show significant pattern differences at some elevation angles.

http://www.freeimagehosting.net/3u74b

R. Fry
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GM7CXM
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« Reply #6 on: April 22, 2013, 12:41:23 PM »

R. Fry: Many thanks for your input, I was expecting the pattern to be a bit more distorted than it is and also did not count on the generally higher elevation and lower gain. I appreciate your help!

It may well be that the best solution is to make a rotary version of the garden yagi as Al suggests. I considered this a while back in the process but which as he correctly says is a little heavier and more laborious, the main reason I started looking at the "conventional" vertical beam.

Back to the drawing board. In the meantime my omnidirectional vertical netted me a contact with KH6IB (another Hawaiian!) and ZK3N today, so it's not doing too badly.

73 and thanks again to all for your thoughts.
Duncan EA5ON



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