Bill DeFelice Says It Straight

[R. Fry]

Now I'm conflicted here.. It seems like I've read the statements numerous times over the past year, that buildings obstruct the AM signal, and also never to mount the transmitter where the antenna is next to a wall, and that the antenna must be above the roof line for the signal to propagate properly.

Many popular beliefs are posted about the operation of Part 15 setups that are not based on science/reality.

The reason that MW radiation is little affected by "urban clutter" such as homes and small office buildings/factories and the conductors inside them is because their physical dimensions are small compared to a wavelength in the medium-wave band; therefore their affect on average field intensity radiated from the transmit antenna is very small.  That affect will be larger very close to such conductors, but that affect on the field a few hundred feet away will be very small.

If this was not true, one would expect the coverage radii of licensed AM broadcast stations to have significant reductions in sectors passing through large metro areas -- but they do not.

OTOH, it's not a good idea to install a Part 15 AM transmitter and whip with its base at earth level within 20 feet or so of a grounded vertical conductor.  That nearby conductor will absorb radiation and dissipate it in whatever r-f ground loss is present at its base.

That will reduce the maximum field from that system, but the net, h-plane radiation pattern will still be circular.

[R. Fry]

...the circularity of the radiated pattern of a part 15 AM radiator...And just to be clear... are you saying the part15 AM signal has no problem penetrating solid wall?.. it just doesn't jive with everything else I thought I've come to know.

The answer to that lies in the successful groundwave service that licensed AM broadcast stations provide to cheap, indoor AM receivers located tens, and in some cases hundreds of miles from their transmitters.

[Tom Wells]

However, I once had the transmitter directly on the beach on top the pier & pavilion and tied into to it's elaborate ground systems (consisted of a large array of about 1" copper cables.. You could hear it clearly 7-10 miles away!.. this was several years ago before I realized what a no-no this was!

[

If it's the same transmitter you're considering now, you see what an elaborate structure that is all "ground wire" can do.

It would be very much the same if you had set the transmitter/antenna  "on the sand" and connected to the same ground
if were available within a foot or two.  An efficient ground in an area with high conductivity and low noise probably should
be able to do 5 miles.

[Richard J Powers]

...the circularity of the radiated pattern of a part 15 AM radiator...And just to be clear... are you saying the part15 AM signal has no problem penetrating solid wall?.. it just doesn't jive with everything else I thought I've come to know.

The answer to that lies in the successful groundwave service that licensed AM broadcast stations provide to cheap, indoor AM receivers located tens, and in some cases hundreds of miles from their transmitters.

.. I said a part 15 AM signal

However, I once had the transmitter directly on the beach on top the pier & pavilion and tied into to it's elaborate ground systems (consisted of a large array of about 1" copper cables.. You could hear it clearly 7-10 miles away!.. this was several years ago before I realized what a no-no this was!

[

If it's the same transmitter you're considering now, you see what an elaborate structure that is all "ground wire" can do.

It would be very much the same if you had set the transmitter/antenna  "on the sand" and connected to the same ground
if were available within a foot or two.  An efficient ground in an area with high conductivity and low noise probably should
be able to do 5 miles.

The Rangemaster I have now is not the exact same one I had then (it was stolen), but it is the same model. But yes, I have come to understand that it was the huge  elevated radial system of the pavilions roof (in essence) that was acting as my true antenna, and if I was to connect to that same system at ground level it would provide the same results, and still be illegal.

As interesting as this discussion has been (I really should have started a new topic for it!), but I really don't feel like I've got a clear cut answer.
The patch of ground behind the restaurant is the only feasible ground installation location, but there are numerous problems with it.. it is closed in on two sides by building, there are also in the immediate area above ground gas tanks, a big steel dumpster, and a big steel enclosed truck trailer that never moves, it would involve a very long cable run, and it would be in a prominent area of wandering stumbling drunks destroying it (charming isn't it?).
I would hate to think this is the best option of a legal install.

There is one other idea I've been playing around with.. Simply mounting the transmitter on the roof. as is, with no radials or anything, and grounded via the power cable to the neutral of a wall outlet inside the studio a couple of feet directly below.. this is basically the same way the Talking House transmitters are installed. - I have tried something similar before at a residence about a mile up the road, and was not to particularly impressed with the results, but know that different locations can have different results.
I'm really not sure how effective that might be in comparison to a ground install with radials installed way back behind the restaurant with all the obstructions and drawbacks as described above.

If I can't feel secure that my installation is fully legal, then this little venture is just not going to work, and it's that very reason that has been holding this thing up.
There has to be way.

[R. Fry]

.. I said a part 15 AM signal

The only difference in the Part 15 AM result is the (much) shorter distance to a given field strength contour.

[Tom Wells]

There is one important point you are missing.  If, during your test out on the pavillion, you had a three meter whip
right at the transmitter, a short ground wire to the pavillion as I described before, it WOULD be legal.

And it would work just about as well as the test where you made your connection "up higher" on this grounded structure.

As a massive grounded structure, it would be irrellevant that some of your "ground" current doesn't go
straight down into the ground, but has some circulation throughout the whole structure/circuit.
This creates a field but the more important part is that the whole thing is well grounded over a large area.
The local fields would be contorted by distributed current flow in the structure but as distance
increases the rf field will be more unified.   And so much better than if the grounding weren't so effective.


Would it result in more coverage than (cough) intended?
Well, probably, but that's just too darn bad because it's nobody's "fault" if a true earth ground is way better than average.
An inspector might not like how well it works, but would have to find the combination legal
because  of the obvious connection of a short wire to something that is going right straight into the ground, right there.

Having an obviously grounded ground rod next to the building just outside the window would be ideal.
Is there some bare soil right there by the window?
Set the transmitter by the window, minimum wire out the window straight down to the rod.
Another wire or short as possible coax out the window to the intended radiator, which need not be at ground level,
but could be mounted in an insulated way, part-way up the side of the building. 
Avoid being too close to the incoming AC service (and the vertical conduit which would sap signal as R Fry reminds).


And to mw signals , most walls simply don't exist.
As Rich says, the wavelength is 200 meters, so shorter metallic conductors don't reasonate well nor sap signal.

It might help to think of your mw signal like a single pebble dropped into a still pool.
The concentric circles seen travelling outward ARE the signal, IN the ground.
The locally heard signal is that which has travelled through the ground, even as much as we think the
signal has travelled through the air.

It doesn't really come down through the air at all.  In this case in local coverage, it's sorta rising upward from the earth.

Don't let the skywave concept fog things up.  That's not likely to happen at this power level.

[Richard J Powers]

There is one important point you are missing.  If, during your test out on the pavillion, you had a three meter whip
right at the transmitter, a short ground wire to the pavillion as I described before, it WOULD be legal.

And it would work just about as well as the test where you made your connection "up higher" on this grounded structure.

As a massive grounded structure, it would be irrellevant that some of your "ground" current doesn't go
straight down into the ground, but has some circulation throughout the whole structure/circuit.
This creates a field but the more important part is that the whole thing is well grounded over a large area.
The local fields would be contorted by distributed current flow in the structure but as distance
increases the rf field will be more unified.   And so much better than if the grounding weren't so effective.

Never really comprehending the physics of Am radio waves, Your anology of the pebble dropped in the water does make it considerably clearing to me that the AM waves travel ground and are not an airborne creature.
I've heard it said before, but not in that way, and I think I comprehend the concept more than ever before.

But Tom, as for your indications that my previous install tying into the ground system of the pavilion was actually legal.. well, I've pondered about that several times today after reading your post, and frankly I don't see how that could be true.

The roof of the pavilion is in the shape of a huge octagon, and at each "seam" run the large copper cables which all meet at the pinnacle of the roof, with the other ends all extending down and deep into the sand at all sides only a few feet from the Atlantic ocean.
I had my transmitter imeadiatly above the pinnacle and connected to the cable system with about a 4 inch piece of wire.
Therefore by definition of this setup my antenna system consisted of a 102" whip and a very massive, heavy duty elevated radial system. I can not fathom how anyone could look at that and consider it a legal part15 installation.

Having an obviously grounded ground rod next to the building just outside the window would be ideal.
Is there some bare soil right there by the window?
Set the transmitter by the window, minimum wire out the window straight down to the rod.
Another wire or short as possible coax out the window to the intended radiator, which need not be at ground level,
but could be mounted in an insulated way, part-way up the side of the building.
Avoid being too close to the incoming AC service (and the vertical conduit which would sap signal as R Fry reminds).

There is absolutely no dirt anywhere near the window or the front of the building, the only window isn't the kind that opens anyway, but a solid sheet store front type window. The closest dirt is in the narrow ally between, and about 15 feet back from the front. The office is tiny, only about 16' square or so, with one door and a small window unit air conditioner embedded in the concrete on the alley side wall. There's not even a bathroom.
Even if I did find a way to put the transmitter inside, it would probably reek havoc with the wireless digital phones and fax in the office, as well as screw with the tv reception.. I don't know if the 5' foot high steel safe would have any effect on absorbing the radio signal or not..
But it's all besides the point.. I doubt having my transmitter inside the building would be the best option, and although I'm quite naive about radio engineering, I will continue to consider it the worst possible solution, until proved otherwise.

I appreciate the discussion, but the idea still sounds ridiculous to me.

[Tom Wells]

The point I'm trying to make is that the actual point of conection upon the pavillion is what would make the
difference.  Attached to the metal 1 inch from the sand, with a 1 inch wire and you're golden.

Connecting further up on the metal invites the possibility for the inspector to count that additional distance as part of the
total circuit, and consider how much branch current might be flowing and then declare an extended area to also count as
part of the total circuit.

In this specific case it's important.

It would not matter that the ground is also tied to an above ground structure, so long as your point of connection is
clearly intended to be as close the actual earth as practicable.


That office sounds ideal to me.
No opening window?
No reason not to squeeze a coax right past the airconditioner. 
If you were able to attach ( and tune ) a 3 meter wire right at the transmitter, with the first 2 feet being
"wasted" passing through the wall, then turning upward to finish out, you'd still do better than you might think.
Insulated wire, naturally, to make sure the airconditioner does not become an unintended branch of the circuit.
Oh, it will anyway somewhat just by having the wire against it, but that's not really going to make much difference either.


It's worth seeing what happens with just the grounding at the power socket.
If the wiring is through conduit, there should be a somewhat "OK" rf ground.
How far away in the building IS the power entry and circuit breaker panel?
Sounds like it couldn't be too far away, right?
If local codes require a ground rod at the service entry, and the soil is wet, you're good to go.

Then, too, that air conditioner drips onto soil, so that's where a ground rod really should be, and then
it would be directly beneath the antenna, which would be above the airconditioner.
Maybe the short coax to the antenna mounted ON the airconditioner.
Its case should be already be grounded, and even with the separate wire to a ground rod below, this
proximity of the base of the antenna to a ( even though separately wired ) ground would probably help.

With 100mw at AM radio wavelengths there will be no interference or interaction with wireless digital phones, faxes, TV, or FM.
No, the safe won't make any difference.

It might be possible for some weak AM signals to be intermodulated (mixed)with the pt 15 AM signal...within 30 feet or so...
And that would be hit or miss by frequency combinations (of signals) that may or may not exist in your area.

[pianoplayer88key]

So you have your transmitter connected to a 2.95-meter antenna and a 5 cm ground lead (the end of which is apparently "visually") at earth ground.  Actually, though, that "ground lead" extends to a buried cable underground, which goes maybe a few dozen meters (or maybe 1/4 or 1/2 wavelength) then is connected to a 1/2-wave tower, which itself is grounded with the standard 120 x 90° radials (or, if possible, a Franklin (1/2-wave on top of 1/2-wave, insulated and "fed" at the center)).  What would the situation be in that case?  Or, if you weren't connected to the taller tower, but somehow spaced it so it was acting as a parasitic re-radiator, then what might that be like?  Since your part 15 transmitter is supposedly "properly" grounded, would the FCC still bust you?

[Tom Wells]

So you have your transmitter connected to a 2.95-meter antenna and a 5 cm ground lead (the end of which is apparently "visually") at earth ground.  Actually, though, that "ground lead" extends to a buried cable underground, which goes maybe a few dozen meters (or maybe 1/4 or 1/2 wavelength) then is connected to a 1/2-wave tower, which itself is grounded with the standard 120 x 90° radials (or, if possible, a Franklin (1/2-wave on top of 1/2-wave, insulated and "fed" at the center)).  What would the situation be in that case?  Or, if you weren't connected to the taller tower, but somehow spaced it so it was acting as a parasitic re-radiator, then what might that be like?  Since your part 15 transmitter is supposedly "properly" grounded, would the FCC still bust you?

Well, in that case the "real" rf ground would be over at the base of the grounded tower where all the radials meet.
That would be the point of least rf impedance.
If the buried cable were insulated, then the ground is remote.  Now if you attach to  an end of one of the radials, of
uninsulated wire, you might get by an inspection. 
Whether any meaningful current would flow on any tower beyond such a grounded line is hard to say.

Maybe if it appeared that the ground was to copper ground rod, and only a little bit of it were grounded (for show), and then the buried cable went over to a tower, it might work well.  Depends on whether the inspector pays any attention to the larger tower.
A "leak" anywhere along a transmission line or tuned element is not worth the loss.

It's think it's probably worth trying to keep the intended ground as close to the point of the antenna as possible;
having a large offset in distance can only distort the pattern.

My own situation includes a 100 foot metal light pole about 50 feet due east of the house, and it seems to behave as a reflector;
my signal seems to favor the west, through buldings, rather than to the east with open parkland.