online calculator to estimate range at the MW band.
A quick google keyword search of "online radio frequency propagation calculator" resulted in quite a few results. Here is the top result with a bunch of stuff! And free!!
http://www.educypedia.be/electronics/electroniccalculatorrf.htm
RFB
Lets assume I can really measure field strength with my SW radio. (see below) Lets assume at 3 bars going to 4 bars happens at 90 uV/m (-68 dB), and that is what I see at 40 meters. One of the calculators for isotropic path loss says a 7.28 dB loss at 40 meters for the 1380 Khz Freq. Adding them I get -60.7 dB or 0.001 uW. Lets assume the antenna is getting 100 mW. That does not sound right. That is a percentage efficiency of suck, 0.001%.
Some calculations sent to me said that the antenna is about 0.038% efficient. I have to believe it is at least 1% efficient to get 4 bars on the signal strength meter at +130 feet from the antenna.
Using my SW radio's signal strength LED bars as a field strength meter may not be accurate apparently. I was guessing at -68 dB or 90 uV for the top of 3 bars/bottom of 4 bars. I did this by plugging in a HP signal gen into the SW radio with a 50 ohm load. I turned the attenuation down to the point where the full 4 bars went to 3 bars. I re-did this test again inductively coupled with a small ferrite bar wrapped with wire (made to resonate in the MW band), sitting on the plastic case where the internal loop-stick antenna is. It gave about the same reading. It was consistent.
I assume I can walk away from the antenna and when the signal goes from 4 to 3 bars I am at -68 dB? May be the signal is much stronger. However the real confusing thing is the FCC. They say for this Freq, 1380 KHz the field strength should be:
24000/1380 = 17.4 uV at 30 meters.
I am another 10 meters out and 5 times that. I just don't have a feel for antenna efficiency (with these short antennas) and milli watts. The antenna I have is a full 3 meters tuned by the "Automatic Tuning AM Transmitter" antenna tuner inside the TH transmitter. It is hanging against the house with vinyl siding. It looks like it is time to make an RF field strength meter.
Using my SW radio's signal strength LED bars as a field strength meter may not be accurate apparently.
Considering the FCC field engineers use a Potomac meter pricing at 6 grand + a piece and the calibration is about almost half that, I would safely say that any standard off the radio store shelf is not going to have the highly defined accuracy of one of those Potomac's.
As to antenna efficiency, well there are a number of things that can affect those numbers to which the calculators cannot take into account unless there are entries for that data. Things like nearby conductive objects, inductive objects like natural growth elements such as plant life, ground conductivity and co-efficient, atmospheric moisture content and tons of other little things. Ball park figures are about the best you can expect. The current mounting configuration with the antenna up against a wall will no doubt shunt most of its signal in the direction facing away from that wall..and is probably why you get 5 times the reading at 10 meters more distance.
The antenna I have is a full 3 meters tuned by the "Automatic Tuning AM Transmitter" antenna tuner inside the TH transmitter. It is hanging against the house with vinyl siding.
See here is one example of some factors not taken into account. How much of that signal is being absorbed by the vinyl siding, the wood and drywall material, insulation and internal wiring. All of which will inductively affect that antenna's efficiency by a large margin. Maybe suspend that antenna off a flag mount for a wall and a stick about 5 feet long so the antenna is some kind of distance from it. Having it right up against the wall like that will seriously hamper with the actual efficiency as well as the numbers and readings on a field strength meter.
Counter weight it with a fishing line sinker at the bottom to give the wire some stability and rigidness and anchor it with fishing line to the ground tied to your ground rod or ground system. Then take another set of readings on the SW receiver.
RFB
The current mounting configuration with the antenna up against a wall will no doubt shunt most of its signal in the direction facing away from that wall..and is probably why you get 5 times the reading at 10 meters more distance.
Well I took your advice and have the bottom of the wire antenna way from the house, like a 'slope-er' of about 20 degrees. I secured the bottom with a short lanyard to a 3 foot tall small garden wood fence. Looks like I planned it. I don't know if it's a placebo effect but the signal is slightly stronger outside, not a quantum jump. However in the back bedroom on the 1st floor, nearest the antenna outside, the SW radio is overloading in that room. I have to switch to LO (about a 10 dB cut). I don't think that was the case before, so the signal in the house went up, a good thing. That's the main audience, me, my self and I. The flag pole idea is a good one. However I'm already collecting parts for an antenna, and that kit your recommended looks like a winner. I think I'll be planting an antenna/transmitter in the garden this spring. I noticed RF systems has nice aluminum poles, might be nice, verses copper. I was at one time thinking of using 1-1/4 copper. Then I found, it was $9 a foot. 3/4" copper is plenty, and sure is more convenient.
Aluminum is a better choice as it is more resistant to corrosion than copper. As corrosion develops on the surface of the copper rod, that affects the signal emitted because the signal applied rides on the outside surface of the rod, and if there is corrosion, or the typical discoloration from the shiny copper color to a dull bleh color, that will reduce the effectiveness of that signal being able to ride on that surface as well as become somewhat blocked by that surface corrosion. Aluminum does not suffer from this effect.
Thats great that moving the antenna a little at the bottom resulted in a positive mesurable improvement. It will most likely improve even more if the top section is also moved away from the wall. This should show you clearly that with the antenna up against the wall, the wall and its materials were absorbing much of that signal. In effect, the wall was acting like a director even though it was also absorbing the signal. With it away from the wall, the emitted signal pattern has become more symmetrical..ie more omni-directional from a vertical element.
Of course the larger the diameter of the emitter pipe, the better the sound quality will be in the signal. But a 3/4 inch copper or aluminum pipe will give you a good clean solid sound. You will notice a good difference between the wire antenna and the 3/4 inch diameter pipe!
With the antenna idea you plan to build for spring, I would suspect that your audience count will increase dramatically! 🙂
RFB