roof top installation question

Soundcrafter,

Welcome to the site and don't hesitate to participate.

Your stated goal of learning is great and your learning will only happen if you try this and see what happens. My offhand prediction is that if you "ground" the transmitter to the roof installed as you described you will gain range. Connect the transmitter circuit ground to the metal roof. There will be no problem with routing the antenna lead through a hole in the roof. At AM broadcast frequencies the capacitive reactance caused by the wire through a small hole is of no consequence. The metal roof should provide a good electrical "ground" for the antenna to work against. Try it and let us know what you observe.

You should achieve a greater range gain if you use a loading coil and modify the transmitter per the instructions, but I think you should try one thing at a time.

You may be aware that the FCC rules limit the total length of the antenna, transmission line, and ground lead to 3 meters. This raises questions as to what the "ground" and "ground lead" is. This has been extensively discussed and debated on this board to the point that continued discussion about this is not encouraged, but you might want to do a search on this site for previous posts on this topic for your own information.

Neil

Thanks for the reply Neil.

The power that I have available in the barn loft is tapped from a light bulb so there no third pin ground. How do you think I should configure the rf choke pins on the SStran?

I am hoping to have some time to play this weekend. Do you think that an 8' copper pipe would radiate better than a wire? (I guess that one is easy to answer...) Do you think the SStran can tune to it acceptably without the circuit mods? The roof itself does not appear to be grounded. How would it change things if I ran a ground strap from the edge of the roof to a ground stake? I guess I should do some checking of the different sections of the corrugated roofing to see that they electrically connected. Now I just need to figure out how I can mount an insulated pipe in the center of a sloped metal roof barn without spending money!

I know that some of this might now be in total part 15 compliance but I am not worried about that since I don't plan on doing any consistent broadcasting and have only a few neighbors that could possibly be in range under the best circumstances. (I sometimes drive a few miles over the speed limit too but not enough to get stopped if you get my drift)

I spent 20 years in the Radio business mostly in programming but did some engineering too so this really is just for fun. At the same time, I really believe that the FCC needs to address the issue of low power broadcasting in a way that allows the average, responsible citizen to have access to an easily licensed way to serve the community even if it takes on a commercial stance. The AM band is such a dinosaur of radio these days that a low power (but usable) service would present absolutely no threat to the commercial conglomerates that have homogenized radio but would allow some programming diversity and actually local service!

Thanks for all you folks do!

You are in an enviable position by having a large metal roof to use as an elevated ground plane. If I had a metal roof, I would not run a wire from the roof to earth ground because it might be considered to be a "ground lead" under the Rules.

The upper end of the AM broadcast band is fairly quiet in rural areas during the daytime. While your neighbors are more distant than in the city, man-made noise is also lower. This might make up for having distant neighbors.

A pipe is a better antenna than a wire because it has more capacitance to ground than a wire. The higher capacitance reduces the loading coil inductance needed for resonance, and this reduces the losses in the antenna circuit. For the same reason, a larger diameter pipe is better than a smaller diameter pipe.

Soundcrafter,

There is no need for a third ground pin on the AC source since the unit uses a two prong wall wart.

How do you think I should configure the rf choke pins on the SStran?
Start with them installed. If you get hum or other artifacts, try removing them. Choose the configuration which works best. Also be aware that if the GAIN control is set high you will probably get hum.

Do you think that an 8' copper pipe would radiate better than a wire?
There will be a slightly more narrow bandwidth with the wire than the pipe but you shouldn't notice any difference.

Do you think the SStran can tune to it acceptably without the circuit mods?
The only sure way to know is to try it. I found with my unit that the switch settings in the book were a good starting place but got better performance with other settings. If you use a loading coil in the future you will need to make the mods.

How would it change things if I ran a ground strap from the edge of the roof to a ground stake?
If the roof is reasonably large this may not make much difference but there could be lightning safety issues if it is not grounded (and even if it is). Usually, a ground wire from the earth to an "elevated ground" increases radiation but it does so in a way which may not be legal.

At the same time, I really believe that the FCC needs to address the issue of low power broadcasting in a way that allows the average, responsible citizen to have access to an easily licensed way to serve the community even if it takes on a commercial stance.
That would be nice.

Beyond some general comments I have not given much advice. What works here for me may not work for you so it will come down to you trying your different ideas and going with what works best.

Let us know what works and what doesn't.

Neil

1. Do install the jumper on in the audio circuit, otherwise you are likely to get a lot of hum (see posts on my blog). The other jumpers are optional; I have not seen a lot of difference one way or the other if you have a good ground attached.

2. You can try the internal matching network to get started. It will easily tune any wire or pipe of 3M length. The efficiency is not the best since the internal matching coils are all very low Q, but it does work.

3. Why not try grounding the transmitter to the metal roof, but instead of feeding the antenna wire through a hole in the roof, just let it hang down inside the barn? This will be simple to install and I think you will find it will work pretty well. Run any cables horizontally away from the antenna wire and let it hang free (you can attach a small weight to the end to make it vertical).

4. I think the SSTRAN is an ideal way to get started and if you adjust it properly, the performance will astonish you! With your elevated ground plane I would not be surprised if you obtained a range of a couple of miles on a clear frequency in the daytime. I have written about how to adjust the audio controls -- check my blog.

5. I prefer the pipe over the wire, but actually I doubt you will notice any difference at all unless you use an external high Q loading coil. And even then, the difference will mostly pertain to audio frequency response, not RF performance. If you use the internal matching network, I doubt that it would make a noticeable difference based on my experiences, because of the very low Q of those inductors (which de-Q's the antenna circuit).

If you really want to boost the range, you'll have to go to a higher Q loading coil. You probably have plenty of room inside the barn for a large coil.

WEAK-AM
Classical Music and More!

I might add that, if there is no ground wire from the metal roof to a ground rod, there will still be an RF current path to the earth. Depending upon the area and height of the roof, there will be several hundred pF of capacitance to ground. The same situation occurs when using a vertical antenna mounted on a car. There will be capacitive coupling between the body of the car and the pavement.

Note added 8/10/08:

Weak's proposal of an inverted vertical antenna hanging down from the metal roof is very interesting. This kind of antenna will radiate, but poorly. In the antenna circuit, the displacenent curent flows from the vertical element to the roof, and from the roof to the ground. These displacement currents flow in opposite directions, producing a low radiation resistance and high antenna reactance. The performance is much better if the antenna is pointed in the upward direction from the roof.

I suggest you try it before concluding that it will work poorly. Hams have been using inverted ground plane antennas for years, with good results.

I am always surprised how many things people "know" won't work, so they never try them.

However, I would certainly be interested in a comparison report, if anyone tries both configurations.

WEAK-AM
Classical Music and More!

A short ground plane antenna of the usual four-radial type works nearly the same right-side up as upside down. It all depends upon the specific geometry used. You are correct that experiment is the final arbiter.

Hi guys. I tried several options and thought I would share some results. All of these are with an unmodified SStran. I am not trying to become a community radio station due to my rural location. This really is just a hobby but lots of fun. When I say rural I mean that there are only 4 houses on my street and we all have several acres of land and the other side of my road is about 2000 acres of pasture but at the same time I am just outside of a huge metro area with all the associated noise and mass of signals. The only frequency close to clear is 1650.

I tried running the supplied wire out the front of the barn loft to a joist that holds the weather vane and then tried some grounding schemes. There are actually two sets of metal roofing. One covers the loft and the other covers the stalls about 4 feet lower and out further. Neither of these are actually ground to earth. I tried both for the ground and the tuning voltage went way down and I could not get it to tune up well at all. It was actually better with no ground at all but coverage was maybe 50 feet. When I extended the ground wire down to a cold water pipe (yeah, I know.... I was not in compliance) it got a little better but still not too great. The audio was pretty bad too with a hetrodyne sort of squeal in there. I think one thing that was really messing things up was the copper weather vane in very close proximity to the antenna wire.

Next I tried a couple of configurations from the house. One was with the transmitter inside next to a window (non metallic window and screen) and ran the antenna wire up a wooden stick attached to the side of the house to get it up some and the ground wire out the window to a pipe. This was considerably better. Tuning was good. sound was good but the signal was very directional. Range overall was maybe 100 feet.

I tried the same sort of configuration but this time put the transmitter and Mp3 player on the roof in a plastic box and ran the wooden stick up the chimney so that it was now above most things but also near a TV antenna. I know that using a wooden pole is not a good plan but I thought it would be OK for a quick experiment. I ran the ground wire down to a cold water pipe. (yeah, I know but it was only an extra 4 feet of wire or so to get there.) This was the best setup yet. Again tuned up nicely and the signal was usable to about 200 feet. I think the biggest advantage was height.

After looking at some of the antenna plans around and looking at the current price of copper pipe I decided to pick up a Radio Shack 102" CB whip. I first tried having it attached to a metal speaker stand (the aluminum kind that telescopes up for PA speakers) with a piece of plastic conduit for insulation. I got it up was high as possible next to the house with the cold water pipe underneath so that I would be pretty much in compliance. The transmitter was in a box hanging on the stand. This was the best setup yet. Tuning was solid, audio was good and the range was now about 500 feet.

Next I managed to talk my wife into letting my take down the copper weather vane from the front of the barn and replace it with the 102" whip. (she is sure that I have gone crazy and she had better just play along with me until I get better) I put the transmitter in the loft on a shelf right inside from the joist holding the whip and ran a short lead out to the whip and clipped the wire to it. The whip is shorted a bit due to going down into the hole where the weather vane was. I again tried ground to the roof (actually both roof structures) and found the same situation as before with the wire. Tuning went way down. I tried several switch positions on the load circuit but none were good. I then tried actually grounding the roof to the water pipe but this did not help and seemed to make things worse. I am guessing that there was just too much metal for the internal tuning unit to handle.

With the whip still in the wood joist from the front of the barn but rising above the metal roof, I ran a ground wire down to a cold water pipe. (yeah, I know...) This was absolutely the best yet. Theu tuning of the transmitter was pretty good but the voltage was not as high as with just the wire antenna. I was getting out a good 1/4 mile or so on a cheap radio (in my VW) and over 1/2 mile with a decent radio (GM truck). Now, when I refer to the range I am speaking of an identifiable signal not necessarily very clean. The audio was good, the pattern was pretty non-directional. I have some huge power lines to the north of me and they seem to block the signal that direction but other directions seemed pretty equal.

So, in an attempt to be more compliant, I ran grounded power up to the loft and connected the ground lead to the power ground wire. This did not seem to effect things much. Still tuned up decently and the range is about the same. It would seem like a compliant setup. The transmitter is on a shelf at the highest point in the barn with a 12" lead going out to the whip that is now about 97" and about 8 inches of wire to tie to the power ground.

While I am really satisfied with this setup I am still planning to try building a loading coil and do some more experimenting. I was thinking that I could have the loading coil on the shelf with the transmitter and have it's output go to the 12" lead going out to the whip. This would make it where the coil would not have to be water proof and simplify the whole setup as opposed to trying to mount it outside literally at the base of the whip. I have built one temporary coil using 18 gauge stranded hookup wire wound onto the plastic round case that 100 blank CD's came on. It is 5 inches in diameter and I managed to get about 60 turns on there with 5 taps in the last 20 turns. I don't know how it will work but it is worth a try. When I find some more time I will do the mods to the SStran an give it a go. I know that magnet wire and PVC would be better but I was just trying to make due with what was around the house.

I'll report back soon with some more results but would also welcome any thoughts, ideas and suggestions.
(I'll pass on the crazy arguments about grounded elevated poles though.... does it remind anyone of the pharisees and Jesus?)