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Author Topic: Antenna conundrum  (Read 5336 times)
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WB2EMS
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« on: February 11, 2020, 02:53:25 PM »

Hi,

I've been playing around down on 630 meters and having some fun. Enough fun to acquire a class D amplifier to run about 200 watts, up from the 8-10 I can manage out of the ANAN200D. I've been using a dipole antenna, built from electric fence wire and insulators, running about 900 feet in the woods behind the house. It's only 8 feet up, and fairly sensitive to the ground underneath. As it changes from mud, to fresh rain and standing water, to ice, and then a foot of snow, the resonance point moves around quite a bit, about +/- 10 khz, which is enough to raise the swr at the operating frequency up to about 4:1. I'm making these measurements using an AA-1000 antenna analyzer which is rated down to 100 khz.

The class D amp came with a LPF and 'scopematch' device, which through a transformer and some caps samples the voltage and current to the antenna for display on a scope. When all is well, the two traces lay on top of each other. Otherwise ELI the ICE man moves the traces out of phase and shows the difference in amplitude. I'm warned that the amp *really* doesn't like mismatches so I'm trying to get the antenna match as good as I can.
http://njdtechnologies.net/k5dnl-class-d-digital-mode-amplifier-and-accessories/

I can't deal with the differences in ground conditions day to day, or hour to hour, by trimming the antenna. One day I'll be taking a couple of yards off, the next adding a few on. So I thought I would try to do it by using an antenna tuner. Obviously most tuners don't do 630 meters, but I've got an old TenTec 238 L network tuner that goes down to 160 meters and I'm not trying to match an extreme case, so I thought that it would probably get me closer if not spot on. And that seems to be the case.

Over the course of a couple of days I found I could tweak a match going into the AA-1000 from the tuner down to very close to 1:1. Should be good, right? But when I attach the LPF/scopematch to the same port on the tuner, and try to use the amp at low power, the scopematch shows things to be out of phase and of differing amplitudes. When I swap to a dummy load on the output, the traces line right up as they are supposed to. http://k5dnl.com/scope200.htm. When I put the AA-1000 analyzer on the input to the LPF/scopematch, the SWR also looks bad, about 4-5:1. Put the AA-1000 back on the input to the tuner and we're back to 1:1. WTF?

I've pulled the scopematch assembly out of it's box to look for obvious issues and don't see any. No parts damaged or loose from shipping. I reflowed some solder joints just in case with no change. It behaves well with a dummy load on the output, so I'm tempted to think it's working ok. But it gives a really different view of the load presented at the input to the antenna tuner than the antenna analyzer does. And I'm not sure why.

I'm also wondering why the analyzer gives a different result on the input to the LPF/scopematch than it does when moved to the input of the tuner. Why would those change the impedance? The filter should be flat at that frequency and the scopematch is just a sampling device on a passthrough like most SWR bridges are.

I've been wondering if somehow I'm measuring at different freqeuencies due to harmonics. Perhaps the AA-1000 output down at the 630 meter band isn't clean, and I'm matching some prominent harmonic in it's output rather than the base frequency. That seems unlikely, it is rated to operate down to 100 Khz, but not impossible. I've got an AIM4170 analyzer that I'll try this week to see what results I get from that. Or maybe the output from the amp is dirty, but I think that's a given - the amp is class D, followed by a LPF in the LPF/scopematch assembly. What comes out of that combo to the tuner should be pretty clean.

So I'm left scratching my head and unable to use the amp because I can't get a good match for it. I'm thinking about building up a tuner just for 630 meters, probably an L match with a roller inductor or maybe a big piece of air dux and some clipleads and the biggest cap I can find just to experiment with matching. Not many folks run a (sorta) resonant dipole on this band, so not much in the various web sites on matching it. Most work a short vertical or marconi T with a big loading coil built on a 5 gallon bucket - which I may move to once the shack gets moved to a more permanent location but for now I'm going to be using the dipole.

Any thoughts as to why I'm getting different results on the scopematch vs the analyzer? Or why the input to the LPF has a bad match showing on the analyzer? Any thoughts on homebrewing a crude tuner for the band to fine tune the antenna?

 
Thanks in advance.

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73 de Kevin, WB2EMS
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« Reply #1 on: February 11, 2020, 07:54:46 PM »

I have no clue regarding your measurement disparity.

I wonder if laying out a wire on the ground underneath the dipole would stabilize the variations you get due to the weather effects on the soil. I would imagine a good conductor would have a dominant effect and minimize the soil effects. Yeah, it's another 1000' spool of wire and something to trip over, but that's my only idea.

I think you're in uncharted territory with this one.
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« Reply #2 on: February 12, 2020, 10:07:29 AM »

The idea of laying out a wire to stabilize the variations is a good idea and wouldn't be hard to implement. Just another $25 roll of aluminum fence wire. I wonder if I should lay it on the ground (will get covered with snow but be less of a trip hazard for the bambis) or maybe space it up on insulators 6 inches or a foot.

Thanks for the idea!

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73 de Kevin, WB2EMS
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« Reply #3 on: February 12, 2020, 10:28:06 AM »

The idea of laying out a wire to stabilize the variations is a good idea and wouldn't be hard to implement. Just another $25 roll of aluminum fence wire. I wonder if I should lay it on the ground (will get covered with snow but be less of a trip hazard for the bambis) or maybe space it up on insulators 6 inches or a foot.

Thanks for the idea!


No guarantees that it would work. I can't point to experience or data for the idea. It's just a thought.

I think you're working at a disadvantage using horizontal polarization on 630M. If you have the room to install a vertical, it would be interesting to compare it to the dipole... even if only on received signals.
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« Reply #4 on: February 12, 2020, 01:34:39 PM »

I have a bunch of trees around at about 40', so I could get some sort of short vertical up with some top loading wires - I know the dipole is not very good at low angles, but I've been doing surprisingly well with it, heard out to the midwest and down into the Carolinas. Not too shabby for 8 watts TPO. Adding 16 db of power should get me to close to 5 watts EIRP, mostly straight up, but based on how I've been copying some more distant stations it should let me have a two way with them. Next year I might have a tower that I can do something with to get more height, but putting up the dipole only took a few hours over a couple of days and 'good enough is the enemy of better' if I can work out the slight tuning issue somehow. All the folks who use verticals have to retune using their variometers and impedance taps so I'm not alone in the problem with ground conductivity changes.

I do plan to try some vertical stuff at some point. Right now, having a 5 gallon sized coil with high voltage on the output won't blend well with my current temporary shack setup. I could remote it but that's another whole bunch of development. Tuning the dipole is probably easier for now. 

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73 de Kevin, WB2EMS
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« Reply #5 on: February 12, 2020, 06:19:40 PM »

Interesting stuff. If slight changes in the soil make that much difference it shows that it must be considered as an important part of the antenna system. The near field must be very large at that frequency. A single wire on the ground under the dipole, as I suggested, might not make much difference. No way to know without trying.

The common thing with antennas for that band seems to be wire... lots and lots of wire. Wire in a ground system, wire in a top loaded vertical, wire, wire, everywhere. If putting a wire on the ground doesn't help it will probably be repurposed in the future for a vertical installation. You could even use that wire to make your dipole into a folded dipole. That would raise the feed impedance by a factor of 4. Would that be useful? I don't know. Either way, I see a lot of wire in your future. Have fun. Smiley
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« Reply #6 on: February 17, 2020, 11:28:20 AM »

So I walked the antenna system on Saturday and found two issues. One was the wire was broken about 300 feet from the feed point, apparently by a leaping bambi based on the hoofprints crossing below. I dug the missing end out of the snow and reattached it with some splicing wraps and got it back up on the insulator where it belonged. It's kind of puzzling though because the resonance had moved way *down* in frequency. I'd have expected a drastically shortened antenna (missing 150') to go up instead. Hopefully the bambi won't hit it again anytime soon.

The bigger problem was at the feedpoint. The feedpoint has the coax coming to the tree and transitioning to the two halves of the dipole at about the 5 foot level, with parallel lines going up as high as I could reach with the ladder before I abandoned using it, then angling off to each leg of the dipole. I needed a way to join aluminum shielded RG6 to the aluminum wire and decided to use some nice copper clamps I got at Homely Despot. These have a bend in them and a nice little hole that was just looking to be attached to something to hold it all nice and securely, so I passed the nail for the fence insulator through them and through the insulator and INTO THE TREE!  Shocked  I don't know what I was thinking when I did that. I plead hypothermia. I must have just looked at the pretty yellow insulator with the clamp on top and though it was insulated.

So essentially I had about 3 inches of live tree between my feedpoint connections. No wonder it looked ok at milliwatt level with the antenna analyzer, but went wonky whenever I applied any significant power.  Doh! Probably also explains the swings in swr seeing on a daily basis as the sap was rising or not. There may still be significant changes with the ground conditions, that remains to be seen, but I'll bet a lot of it had to do with the conditions between those nail points in the tree.

I pulled the nails out of the tree and left the connections  hanging after reworking them a bit for better connection. Will need to work on making it a much cleaner and solid connection in the future. Walked some more of the wire and knocked away any branches that had moved to make contact. I still have about half an antenna to check out that way, but it was getting dark and I was getting cold.

Went back inside and checked the resonance with the antenna analyzer and it was at 475 khz as it had been trimmed to before. Put the amp and LPF/scopematch on and tried a low power transmission at about 50 watts and it worked correctly. I ran the power up to 200 watts, 300vpp, and it stayed stable! So I think we have fixed that conundrum. Made a few JT9 contacts and also did some WSPR and got heard further than ever before, down into New Mexico.

I'm sure there will be other challenges. But this was a big uplift on my 630 meter quest.


* doh.jpg (1780.86 KB, 960x1280 - viewed 399 times.)
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73 de Kevin, WB2EMS
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« Reply #7 on: February 17, 2020, 12:05:32 PM »

Now I feel a little better about some of the silly things I've done before. Smiley
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« Reply #8 on: February 17, 2020, 03:39:19 PM »

Yeah, this one was a doozy.  Grin  I don't know if it's a harbinger of things to come, or a high water mark and warning to pay more attention. It was cold, near zero, and getting dark, and I was cold when I did it so I was a bit rushed and somewhat distracted.

That's my story and I'm sticking to it.  Cheesy
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« Reply #9 on: February 17, 2020, 04:53:24 PM »

  Sound like a reasonable story, plausible deniability, after all, all us old farts need to stick together, wish I had the space to play 630 meters.
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« Reply #10 on: February 17, 2020, 05:49:47 PM »

Kevin, Now that this info is in the public domain, prepare for some Sap like the Grandson of Euell Gibbons to come knocking on your door!

I am enjoying this thread.

Jim
Wd5JKO
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« Reply #11 on: February 17, 2020, 09:06:23 PM »


K,


I hope u waterproofed the coax/ant connection.......  woodent hertz to point the coax towards the gnd so the drippage  has a harder time gettin' into the coax.


klc
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« Reply #12 on: February 17, 2020, 10:00:06 PM »

Not knowing anything about 630 meters myself, I've enjoyed your posts.
You make 630 meters look like alot of Fun. When I saw my ARRL chart showing 5W EIRP max,
I thought I would'nt be able to get out very far at that power.
Your running "200W", and making contacts in New Mexico, great.
I'm going to have to check this out and Learn something about it, 630 meters sounds fun..
I've got room for "lots of wire", Keep us posted,
73,
AG5UM
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« Reply #13 on: February 18, 2020, 10:14:44 AM »

Quote
I hope u waterproofed the coax/ant connection.......  woodent hertz to point the coax towards the gnd so the drippage  has a harder time gettin' into the coax.

Not yet, but soon. Mostly we've had snow, not rain here, so not a lot of water to get in but that will change soon. I'm thinking of putting the whole thing in a plastic box. At minimum a lot of 3M  103 tape.


AG5UM, glad folks are enjoying the posts. It's been fun. Last night was probably my most effective night, I made 7 JT9 contacts, the furthest out to about 1047 miles. I heard half of a USB contact, the MD half, but I could tell the FL station was in there and with less noise might be able to copy him. I've got a schedule coming up for a CW contact, and when I get conversant with the mode I want to try JS8call as you can actually talk to people with that - JT9 is like FT8, sort of 'ping' via HF. Somewhat rewarding but not much content.

The 5 watts EIRP limit means that for antennas that have low gain, most are very inefficient, you can run more Transmitter Power Output (TPO). My antenna models out to have -16 db of gain straight up, and less at lower angles that are most useful. So I can run as much as 200 watts into -16 db of gain for a net of 5 watts at the angle of maximum radiation. But even with only 8-10 watts or so I was being heard on WSPR out past the middle of the country. People are making contacts over hundreds of miles with much more moderate setups. If you can, try to give a listen with your existing gear. Generally the setup is to set the dial frequency of 474.2 khz, USB and then use the WSJT-x software to listen to the audio band from 0-3 khz. Most of the WSPR activity happens around 1500 hz, JT9 is a bit below down near 1000hz. USB happened last night at 476.0 khz. But that's fairly rare.   

Useful websites
https://njdtechnologies.net/
http://njdtechnologies.net/what-happens-where-on-630-meters-a-few-more-comments-about-where-to-place-your-signal-depending-on-mode/
https://njdtechnologies.net/category/630-meters/


I wish I had a camera on the feedpoint when the nails were part of the circuit. I wonder if I was making steam come out! (with brief tests at 50 watts).  Grin

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« Reply #14 on: February 18, 2020, 09:11:08 PM »

Thanks for the websites, interesting stuff, I'm going to have to study-up on the subject, 630 meters sounds fun.
Originally, when they gave us the new low bands, being an Antique Radio enthusiast, I thought of the early
1920's tube transmitters like the 1922 Paragon 2-5-u, using two 5 watt tubes on CW & Phone,
It transmits on 180-325 meters,but they made accessories to raise or lower the wavelength.
I would like to try some of the antique "long-wave" transmitters,
They claimed 600 miles on phone, farther on CW.
Anyway, it sounds like your doing great at it, Thanks for the Info. , Have Fun.
73's
AG5UM
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