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Modified Antenna Coil Design

radio8z's picture
Hello All, If you are considering building a SSTRAN antenna, here is a modified coil/mast that I developed. It takes some custom machining, but may save some money on materials. I haven not installed this so it is untested. I present it so you may get some ideas of your own. MORE Antenna Guy Carl advised me against modifying his coil design, so you might want to go with his and SSTRAN's coil design regarding wire and taps. What I present will mechanically work with their coil, but I wound a different coil as shown in the photos and cannot attest to its electrical operation. The major feature, which I think is an advantage, is that the coil is co-linear with the mast and antenna giving a nice appearance and it will eliminate the need for the expensive steel pipe into the ground. I drilled a 3" end cap to accept a 1-1 /2" schedule 40 PVC pipe to 3/4" pipe threaded female adaptor as shown in the picture. I made the hole using a lathe, but a proper size hole saw should work. This should be a centered and tight fit and it is glued in place. With the lathe, I was able to turn a flat on the top of the cap so the hex flange of the adaptor fits flush and provides a good additional mating surface for gluing. Make the internal connection to the coil and copper antenna right the first time since once the caps are glued, it is permanent. Or, as I intend, fasten the top cap to the pipe with self-tapping stainless steel screws driven into the side of the cap and pipe. I soldered a #16 stranded wire inside the 3/4" antenna pipe rather than using a clamp for the connection to the copper pipe. This wire connects to a lead from the coil inside the coil form. Instead of a metal mast at the coil bottom, I use a 3" schedule 40 coupler which fits over a 3' length of 3" schedule 40 PVC pipe which I plan to sink into the ground using a post hole digger. The RF ground will be a set of buried radials. The pictures attached should give you an idea of what I am doing. Note that this is in the design phase and has not been tested electrically or mechanically. I just offer this to you so you can have some ideas and maybe make it better. Neil, Radio8z
As to the inline coil

As to the inline coil design, I had the same thought. I just haven't gotten around to going to Home Depot to see what type of couplers they had. Your method looks good.

I also had a thought about a possible variation. If you ran the copper pipe through both end caps, the result would be much more stable. I worry about the stress of mounting such a long pipe to just one end cap. Will it stand up over the long term, especially with high winds (I live in the SW and Spring is very windy).

Of course, this brings up the question of what the effect of a non-ferrous pipe inside the coil would be. Also, would that additional length be radiating from inside the coil?

If the pipe inside the coil is a problem, you could use a short length of PVC through the length of the coil form. This piece would connect to the copper radiator at the top cap and the mast coupler at the bottom cap. This would be almost as strong and eliminates the possible problems of the copper pipe.

the effect of a non-ferrous pipe inside the coil

[quote=mojoe]Of course, this brings up the question of what the effect of a non-ferrous pipe inside the coil would be. Also, would that additional length be radiating from inside the coil?[/quote]

Great point, and 12vman's experiences with a piece of aluminum, described here:

http://part15.us/node/1162#comment-2966

might give you an idea that it probably wouldn't be good to do that.

HOWEVER, using the PVC section inside the coil portion ought to be fine.

I think there are also a variety of PVC connectors that will accomodate an NPT connection but would require lots of adapter sections to go from a 4" PVC section to 1/2" NPT so just using the pipe cap seems like a cleaner solution.

Experimental broadcasting for a better tomorrow!

Experimental broadcasting for a better tomorrow!

Modified antenna, more thoughts

Mojoe,

[quote]If you ran the copper pipe through both end caps, the result would be much more stable. I worry about the stress of mounting such a long pipe to just one end cap. Will it stand up over the long term, especially with high winds[/quote]

Yes, you are probably right and this may be worth pursuing. A couple of thoughts came to my mind when I put this together. One was to eliminate the need for the steel pipe in the ground since locally the pipe is very expensive ($40 for an 8 ft. length). By burying the 3 inch pvc and installing the coil with a coupler to the top of this pipe, the material cost is low. The second plus is that if water enters the coil through the antenna pipe, holes, or by condenstion it has a place to drain. If one uses a cap on the bottom, it seems appropriate to drill a weep hole.

Though your idea will probably be more sturdy, the question is how good is good enough? Just by fooling around with the assembly and putting lateral stress on the antenna pipe, my guess is that it will fold and bend before the cap will fail. Also, some have told me that UV will make the white PVC brittle over time so I plan to paint the plastic parts and coil with UV resistant spar varnish or other outdoor paint (depending on what my spouse can tolerate). This has not been outdoor tested beyond a quick check for tuning over radials laid on the pavement, and it does tune OK.

SCWIS,

One may be able to fashon adapters but I couldn't find the right ones at the local big box. Since I have a lathe available, I went the route I did. It seems that if one is careful, a hole saw would also work.

I read with interest 12man's flashing tuning experiment. I wonder if the flashing inside the coil acts as a "gimmick" capacitor and changes the coil stray capacitance which could affect the tuning.

Neil

New variation
OK, radio8z has prompted me to get off my butt and go shopping at Lowe's. With the assistance of a very helpful Lowe's employee, I have purchased the following PVC parts for my coil form (not showing the 3" PVC pipe). (Click for larger image) From left to right: 1.5" to 3/4" threaded bushing, 3" to 1.5" reducer, 3" coupler, 3" to 2" flush bushing, 2" to 1.5" threaded bushing. Here is what they assemble to (not glued together yet). (Click for larger image) The left assembly replaces the top cap. The 3/4" threaded copper fitting will attach directly. The right assembly replaces the bottom cap. The 1.5" threaded steel mast will attach directly. Although this requires a few pieces, it is still fewer than the original SSTRAN plans and it does give me an in-line coil. Looking at the thickness of all the PVC pieces, I think the final assembly will be very strong. The cost of the parts shown is about $8. Now I guess I should go cut some PVC pipe and start winding a coil :)
New variation - part 2
Here is what the assembled top and bottom caps look like: (Click for larger image) Note that I had to cut about 1/2" from the bushing that fits into the top cap to make it seat flush. The rest of the coil assembly will be like the SSTRAN plans, so I don't think more pictures will be necessary.
Nice Job

Mojoe,

Nice work and thanks for sharing. This looks great for those who don't want to mess with machining things.

Keep us advised on the progress.

What do you think about using a buried 3" PVC pipe rather than the steel mast?

Neil

Nice Job - Reply

[quote=radio8z]Mojoe,

Nice work and thanks for sharing. This looks great for those who don't want to mess with machining things.

Keep us advised on the progress.

What do you think about using a buried 3" PVC pipe rather than the steel mast?

Neil
[/quote]

Yes, no machining or hole drilling needed. I just had to shorten that one bushing. I used a Dremel, but a hacksaw would also work.

As for using a buried 3" PVC for the mast, that should work just fine, as long as you don't make it too long. Maybe someone else can recommend the maximum length of a free-standing 3" PVC pipe.

Of course, the drawback to using an insulated mast is that you will have to run a longer ground wire to any radial system you have. This long ground wire would be questionable under Part 15. Using the SSTRAN design and extending the copper radiator to its full length, you would only have about 13" left for both the ground wire and antenna feed wire, if you strictly interpret Part 15.

Mast depth

Mojoe,

My suggestion regarding sinking the 3 foot long 3" PVC pipe into the ground comes from the building codes and practices here in the frost belt. I have sunk many poles and built a few pole barns and the practice is to dig the hole about 36" deep and pour a concrete footer up to 32" below ground and place the bottom of the pole on the footer. Since this antenna will have virtually no vertical load, the footer is optional but it is a good practice to sink it to at least the 32" frost line.

If one uses a 3 foot length sunk to 32" there will be 4" above ground which will allow room for the transmitter box to be below the coil and minimize the lead length to ground and the antenna coil.

I have a post hole digger so installing the pipe as I described is easy for me, but for those who don't, driving the steel mast into the ground is a good option and your design will allow this.

My interpretation of the rules requires a ground mounted antenna, but for those who wish to mount their antenna on an elevated mast a metal pipe is more workable than the PVC pipe.

Neil

Completed Antenna

Ok, I couldn't resist a few more pictures. Here is the completed in-line assembly:

[url=http://picsplace.to/][img]http://img6.picsplace.to/img6/24/Antenna_000.J...

Note that the threaded copper fitting isn't flush with the PVC. The PVC seems to be tapered, so I couldn't tighten things flush. Cosmetically, I would have liked to get that last 1/4", but structurally, it is just fine.

And here is a closeup of the base, where the 1.5" mast attaches:

[url=http://picsplace.to/][img]http://img6.picsplace.to/img6/24/Antenna_Base....

Now, I just need to figure out the best way to mount this thing on the roof.

One thing that I would do if I were to build another antenna is to shorten the coil form by 2". Note that if you shorten things, start winding the coil at 2" from the top of the PVC pipe, not the 3" specified in the original plans.

Beautiful Work

Mojoe,

You do beautiful work and thanks for the pictures.

Be sure to let us know how it works when you fire it up.

Neil

These pictures are really, really great

[quote=radio8z]Mojoe,

You do beautiful work and thanks for the pictures.

Be sure to let us know how it works when you fire it up.

Neil
[/quote]

I agree, it's so nice to have something to actually show folks what to build. This might be the first time so much useful info has been compiled in one place.

Looks like the vision of our founder, mlr, is finally coming true!

Experimental broadcasting for a better tomorrow!

Experimental broadcasting for a better tomorrow!

Antenna testing

Thanks for the kind words. As to mounting and testing this antenna, It will be about two weeks or more before that happens. I just ordered another transmitter kit from Phil at SSTRAN. I want to build a new kit and use machined-pin sockets for the ICs. This should help with thermal creep when mounted outside. I don't want to get up on the roof because the chips have fallen out. I also don't want to solder the chips in, to make for easier servicing.

The other reason I want another transmitter is that I want to use 1680 KHz for broadcasting to the neighborhood, but most of my old tube radios don't cover that range. I do have one very nice 1947 Delco radio that goes up to 1700 KHz, so I can monitor the external transmitter with that. For listening on the other radios, I will use my indoor transmitter, set to 1610 KHz. It is a simple matter to feed the audio to both transmitters. Since I am transmitting OTR, it doesn't seem right to listen on a modern radio :-)

Speaking of mounting antennas, I went to Lowe's (where I bought the other parts) and they don't carry 1.5" pipe. The largest they had is 1.25". I then drove to Home Depot and they are closed for remodeling. Just my luck. I'll have to find a plumbing supply store. The other strange thing with Lowe's was they had a nice plastic box to put the transmitter in, but it didn't have a gasket on the lid. The smaller boxes of the same brand had gaskets, just not the size I needed.

When the new transmitter is assembled and the antenna is mounted and tuned, I plan on driving around to check my coverage. I have already printed out a Google satellite map and marked distances to known intersections in preparation. I'll let everyone here know how things turn out.

New transmitter arrived

Just an update. My SSTRAN kit arrived today. I'll get it built and tested this weekend. I'm still trying to figure out the best way to mount the antenna on the roof, however.

Speaking of the SSTRAN unit, this is the second kit I have ordered from Phil and I would like to say that he does a wonderful job with selling and supporting these kits. The service and support are outstanding. Even the way the individual parts are packaged - simple, but tidy. Compared to many others, SSTRAN is top notch.

Almost on the air

Mojoe,

Sounds great!. Let us know what you figure out re the mounting and how it works on the air.

Neil

Re: Almost on the air

Actually, I have been pseudo-operating for over a year. For a lot of reasons (including procrastination), I hadn't gotten around to building an outside antenna until now.

However, that time hasn't been wasted. I have been learning how to use ZaraRadio and some VST audio processing software, cleaning up my OTR collection, putting together a spare PC for automation use and trying to automate other aspects of operation.

During most of this time, I have been broadcasting around the house (and to a few unsuspecting neighbors) with an inside wire antenna, using the first SSTRAN kit I bought.

I can't wait until I get the outside antenna up. Hopefully, I'll get a decent range with it.

Waiting for your report...

I wonder how it all worked out? I was reading with great interest about an antenna design which has the counterpoise and feedpoint AT THE TOP! It puts the radiation where you want it and doesnt rely on a lossy ground! Sounds like just what we need to increase the range to a new level. I may have to try this design when the weather permits(Its 2 degrees with a wind chill of -22 and 3 feet of snow!) Only problem is the elevated counterpoise,I dont know how the FCC would see it,as part of the 3 meter total or what? Even if we just moved the feed to the top it may be worth it. I also saw a loading coil design which uses a moving non-ferrous sleeve to tune it to resonance. It claims to decouple the unused coil windings and their losses,interesting,huh? Regards,Lee

Delayed Report

The reason I haven't posted any results is because I'm on hold, waiting for a metal base to be made for my antenna mast. I should have it this week (I hope). I'm currently renting the house I'm in, so I don't want to mount anything too permanent on the roof.

The antenna is ready to go, the second SSTRAN kit is assembled, tested and mounted in a weatherproof box. I even have the temperature sensor kit working (logging software is still a problem).

I'm anxious to get the antenna up, myself. It just takes a while for things to come together. I also have to coordinate with a friend, as putting up the antenna is a two-man job. I'm hoping this weekend will see things completed.

Preliminary test results

I picked up the metal base last night. Things aren't mounted on the roof yet, but I wanted to try a preliminary tuneup with things at ground level.

With the antenna sitting in my kitchen, I tried a test tuneup. First, I don't have a ground on it at all. Second, the antenna tuning isn't peaked, as it wants to peak with the antenna extended further than my ceiling will allow (about 12 ft).

With this less than optimal setup, I drove around to check range. On my truck radio (which is more sensitive than a table radio), the signal was listenable at about 1/4 mile. In spots, I could hear the signal at 1/2 mile and tell what it was.

This seems encouraging. It will be interesting to see what the coverage is on the roof, with a proper ground and the antenna lengthened for peak tuning. This weekend for sure.

Finally on the roof

Today I installed my modified SSTRAN antenna on the roof. After tuning things up, I fired up ZaraRadio and loaded some old music from 78s. Since I needed to go to the grocery store anyway, this was a good time to test my range.

The following results are with the radio in my truck. The transmitter still needs further tweaking (I'll do that tomorrow).

In any case, I was getting an amazingly strong signal all the way to the highway, which is a mile (or a little more) line-of-sight. When I got to the grocery store (two miles or more line-of-sight), I was still listening to the music, although it sounded like a moderately strong DX station by then.

Needless to say, I am impressed and pleased with the performance of the antenna.

Mojoe Antenna

Mojoe,

The preliminary results are encouraging and I appreciate the update.

I have a question: I couldn't tell from your pictures and don't recall you mentioning if you provided taps on the coil. Maybe I missed it. How did you trim the coil inductance?

Neil

Antenna Coil

The coil was wound and tapped as per the original SSTRAN plans. The only deviations from the original plans were my substituted PVC parts to make the coil in-line. The reason you can't see the taps in the picture is that I photographed the "pretty" side of the coil :-) Tuning was also exactly as per the SSTRAN instructions.

I should mention that the antenna tuned up perfectly. Using the proper tap and the the pipe adjusted for a peak, I was reading almost 15VDC. As per the instructions, I then adjusted the trimmer cap until I had 13VDC. That's as good as it gets. For my configuration, I didn't see that winding extra taps would have been worth the trouble.

Note: In case anyone missed it in the instructions, make sure to use a meter (preferably analog) that has a 10MOhm input. Using a typical analog meter with 20KOhm/Volt input will load the transmitter too much and you will not be able to tune things correctly. Thankfully, I have an old Heathkit FET VOM (and a long extension cord). Otherwise, I would have had to use my Fluke digital. Using a digital for peaking is not easy.

A friend and I will be trying to further improve the range of this antenna. One improvement we discussed is a groundplane on the roof. The only ground at this point is a piece of #6 solid copper wire, running to a nearby water pipe on the roof (evaporative cooler). We might also raise the antenna a bit more. It is now on a 3ft mast, above the roof line. I also know that the modulation level could use further attention.

I did some more driving around yesterday to check range. There are a few minor blind spots, but generally I am getting a good signal out to about 1mi. It is listenable with minimal noise to 2mi. In one location (NNW), I was hearing it DX-wise at 3mi. That was as far as I drove yesterday.

I'm hoping that I may be able to improve things so that I have a solid signal out to 2mi. We'll see. In any event, the SSTRAN antenna design is a good one and does a lot with a little signal.

As soon as I am satisfied that I am getting as much range as is feasable, I will start officially broadcasting. At that time, I'll post a notice in the station listings. The station name that I am using is "Radio Nostalgia" and the frequency is 1680KHz. The format is OTR and period music.

I really love the

I really love the improvements you did to the normal antennas. I think if everybody would focus their attention in this kind of thing we would have lived in a better world by now. But sadly they have other things to fill their time.

Antennas and Tractors

This idea was further developed by mojoe and his design is published on the SSTRAN website.

It happens that I have reconditioned two Ford 8N tractors and have one of them here in suburbia just to ride around the block now and then.

Have you ever seen the Moline UDLX tractor? Sounds like something you did with yours.

Neil

Coil Windings

Does it make any difference whether coils are wound clockwise or counter-clockwise?

Carl Blare

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