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Help with helically wound 160m ground plane vertical

Moleculo

Moleculo

Ham Radio Nerd
Apr 14, 2002
9,202
1,686
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I'm considering building a continuously loaded / helically wound 160 meter ground plane vertical to hide in some trees at home. The overall length would be probably be somewhere around 25-30 feet tall. The idea is similar to the one shown by K6MM at K6MM 160 Meter Vertical - A Restricted Space Helical Vertical! and also discussed in some detail in the ARRL Antenna book under the Short Antennas section of the Low-Frequency Antennas chapter. The book shows constructing a matching network consisting of a couple of variable caps and an inductor (coil), although K6MM omitted that from his antenna. It's just a simple L network and looks like a basic antenna tuner to me.

So here are my basic questions. Is there really any benefit to adding the matching network at the feed point if I already have a good manual tuner in the shack other than mitigating coax loss? If I do go with the matching system, what variable capacitors should I use and where can I get them from?

I'm sure I'll probably have a few other questions when I start building this thing.
 

Tim, I am a great proponent of installing the ATU at the feedpoint. Operating this antenna outside it's narrow point of resonance will result in high losses in the feedline and possibly unwanted radiation from the feedline. In the broadcast business we always had the ATU's located at the base of the towers and matched to 50 ohms there even when running really low loss cable with RF power to spare. If you are not going to run over 100 watts you could install one of the cheaper auto-tuners at the base and be done with it. Using a manual tuner at the base is a pain when wanting to QSY more than 25-30 KHz or so. As for what to use for capacitors, I like the big "bread slicers" but they are $$$ even from places like Surplus Sales of Nebraska. I have picked them up at ham fests for as little as $5 each and never spent more than $20 for one.The one pictured below is a 2000 pF unit I picked up for $15 and the ruler is one foot long. Wider spaced units of about 500 pF or so would be ideal for this project.

2000pfloadingcap.jpg
 
There is another option I have, but I'm not sure if it would work? I have a Mexican fan palm tree in my back yard that has grown to about 40 feet. The base is over a foot wide and it gets more narrow as it goes up. I could easily us this as the support to wrap wire around to make this antenna. The interesting idea about this is that the coil windings would be much wider, making it a little more efficient. Would it work better? I don't know. Does the tree with it's roots in the soil and minerals in the trunk cause problems? Or maybe it would be fun to try this just for the heck of it?
 
Moleculo said:
There is another option I have, but I'm not sure if it would work? I have a Mexican fan palm tree in my back yard that has grown to about 40 feet. The base is over a foot wide and it gets more narrow as it goes up. I could easily us this as the support to wrap wire around to make this antenna. The interesting idea about this is that the coil windings would be much wider, making it a little more efficient. Would it work better? I don't know. Does the tree with it's roots in the soil and minerals in the trunk cause problems? Or maybe it would be fun to try this just for the heck of it?
Click to expand...

pity you ain't got an apple tree in the back garden Tim, run a couple of kw through a helically wound apple tree and you could have precooked apples, just the thing for making a quick apple pie.;)
 
There was a guy that would wrap coils around trees to get them to resonate...

I would definately add some sort of matching network at the feedpoint - absolutely if running a coaxial feed...
 
Moleculo -

I am currently running a homemade helically-wound 5/8-wave horizontal dipole on 11 meters. Since all of my commmunications from here is DX, I don't use vertical antennas so I have no experience with them. My dipole is about 12' long total. I wound each element using #12 enameled wire on a 6' fiberglass fishing pole. I had fair-to-middlin' success initially with RG213 connected directly to the antenna. A dx contact suggested installing capacitance hats on the ends, so I made a pair of 8" diameter disks from copper flashing and installed them. This made a HUGE difference on receive, as the noise went way down. I am assuming it helped on transmit, but I can't prove it. I seldom run over 100 watts dead key. I don´t know if a vertical antenna needs the hat, but I thought I would mention it.

This is my opinion about the matching network... take it or leave it. All of my antennas are homemade since I can't buy ANY amatuer/cb radio stuff here. The antennas all have an acceptable match (less than 2:1) over about 1.5 mhz. Maybe a matcher at the antenna base would improve my 2:1 bandwidth and/or the antenna's efficiency, but I don't think that the end result (a watt or 2 more of power) is worth the extra time and effort. I'm sure the "purists" will disagree with me, but this has been my experience. 73s.

-399
 
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Tim, you must remember that the top of a helical vertical will have VERY high RF voltages present even with as little as 100 watts. I have visions of setting fire to that palm tree in the dry season. The form that the wire is wound on should be a good insulator and I am not so sure a palm would fit that requirement. Any helical antenna will benefit from a capacity hat and the hat will help with corona discharge.
 
The form that the wire is wound on should be a good insulator and I am not so sure a palm would fit that requirement.
Click to expand...

I think the biggest problem would be the questions around how good of an insulator it would be. Palm trees have a significant amount of water in fibers that comprise their trunk. I was actually thinking of a way I could make a large capacity hat / corona disc at the top, but I think I'm going to skip this whole idea of using the tree and do it with some PVC.
 
Moleculo -

The wooden portion of the palm tree might be a great insulator, but I agree with Capt. KW that the water inside the tree will cause a lot of problems. Use the PVC. As a suggestion, I would recommend that you wind the antenna with the majority of the wire wound close to the top of the antenna (like a Pal Firestick). This will make the antenna top loaded and should make for a lower angle of radiation. You can experiment and close-wind the antenna from the middle to the top to see the effect on performance. Personally, I think the top-wind scenario is the best. But at 160 meters, I could be wrong. Have fun.

- 399
 
Moleculo said:
I'm considering building a continuously loaded / helically wound 160 meter ground plane vertical to hide in some trees at home. The overall length would be probably be somewhere around 25-30 feet tall. The idea is similar to the one shown by K6MM at K6MM 160 Meter Vertical - A Restricted Space Helical Vertical! and also discussed in some detail in the ARRL Antenna book under the Short Antennas section of the Low-Frequency Antennas chapter. The book shows constructing a matching network consisting of a couple of variable caps and an inductor (coil), although K6MM omitted that from his antenna. It's just a simple L network and looks like a basic antenna tuner to me.

So here are my basic questions. Is there really any benefit to adding the matching network at the feed point if I already have a good manual tuner in the shack other than mitigating coax loss? If I do go with the matching system, what variable capacitors should I use and where can I get them from?

I'm sure I'll probably have a few other questions when I start building this thing.
Click to expand...

I'm guessing the antenna doesn't have a 50ohm impedance, that being the case then the purpose of the matching network is to match the antenna to the feeder rather than try and match the whole system back at the shack with the resultant coax losses.
 

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