What an eye opener. The idea would be to somehow figure out a way to make a coil that didn't choke off so much power so we could tune up to our antennas. May a larger diameter coil? I know Station8 was trying to work out a better coil that didn't cut off so much RF to the antenna. Its amazing that we honestly can go as far as we do with such little power. But as far as 100mW I think we need to keep that but maybe allow longer antenna and ground and with a transmitter like the Talking House as long as you use the 75 Ohm output that transmitter could grow as technology and rules change. One could connect a linear to it easily if ever more power was allowed. I guess I really have a gold mine here as far as a AM Transmitter. Glad I got it now.
Commercial matching inductors i assume use large self supporting copper tube ?
Rich has previously stated rf loss in the region of 1 ohm or so ?
I plan to wind my own large inductor out of 10mm copper oil pipe, and mount it in an outside waterproof box, will be interesting to see how it performs.
Paul.
Anecdotal statements on the Web have reported values of 15 to 20 ohms for the r-f resistance of the built-in ferrite-loaded coils in some Part 15 AM transmitters, when resonating a 3-m radiator at the high end of the AM broadcast band (3-m whip + very short conductor to a true r-f ground).
An external matching coil might have less loss than this -- but probably not less than 5 ohms unless it was rather large (and expensive). Also any loading coil internal to the transmitter would need to be essentially bypassed to use an external coil.
Usually the resistance in the r-f ground path is the biggest contributor/definer toward antenna system radiation efficiency of Part 15 AM setups. The resistance of an 8' ground rod driven vertically into earth of average conductivity at the base of the transmitter+3-whip usually is fairly close to 50 ohms. (The r-f loss in the buried radial ground system of a typical licensed AM broadcast station is on the order of 2 ohms, regardless of earth conductivity.)
Antenna system radiation efficiency is calculated like this:
Efficiency = Radiation Resistance/(Radiation Resistance + Coil Resistance + Ground Resistance)
Using the numbers in my post above:
Efficiency = 0.1131/(0.1131 + 20 + 50) = 0.001613... , or 0.16%
So to calculate system efficiency using some other values of coil and ground loss, just plug them into the above equation.
Radiated power is found by multiplying the output power of the transmitter by the radiation efficiency of the antenna system. In my post this is 50 mW * 0.00161 = 0.0806 mW, approx.
The fields shown in my post above at 1 km and 1610 meters will change by the square root of the change in radiated power. IOW, if radiated power doubles, the inverse distance fields will increase by SRQT(2), or 1.414X.
As my rf ground system is getting more extensive, and the rf resistance is less than 10 ohm, this now highlights the dominant loss in the matching inductor.
Even though i have reduced turns on the amt5000 toroid, making a large air wound inductor would make a worthwhile difference.
Paul.
Paul,
Your comprehensive development of your antenna system is interesting and informative. Could you describe how you measure the ground resistance of the antenna system?
The system I use is described here:
http://www.part15.us/forum/part15-forums/general-discussion/rf-current-transformer-antenna-tuning
The key to this is the simple RF current transformer which, when placed between the transmitter output and the input to the loading coil, gives a means of measuring the feed point impedance. Resonance is shown by a zero degree phase shift between the V and I and if the coil Q is known the coil resistance can be calculated. From the V and I measurements at resonance the total antenna system resistance can be calculated and the coil resistance subtracted to find the ground+radiation resistance. More details are in the link above.
Neil
Neil,
I have currently based my less than 10 ohm value on the physical radials lengths, and i am now heading for 64 of them, and ground conductivity of 5mS/m.
Online calcs suggest 6.5-7 ohms, but i am sticking to the less than 10 to be safe.
For a while i have been thinking of buying the MFJ 259C, to measure coil inductance, and i would guess, be possible to calculate inductor resistance ?
As the final ground radials will be 64, that will be my final radial system, and then i will proceed with additional top hat testing ideas, then work on a low loss matching inductor.
Paul.
Paul,
Last week I made some measurements on my antenna system. Here are the details. I thought I would make them available in case you want to run this as a check on your calculations. Usually, calculation based on the Wheeler equation for single layer coils is fairly accurate but the Q needs to be measured at the operating frequency.
Radials: 12 each 10 feet long # 12 wire.
Coil: 68 turns #18 enameled copper wire on a 3 inch diameter form plus 5 turns movable trim coil. (measures 254 uH, calculates 246 to 261 uH without and with trim coil)
Coil RF resistance: 18 ohms. (Q = 147)
Antenna system Z at resonance: 32 + j0 ohms.
Ground resistance ~= 32 - 18 = 14 ohms. We have had a lot of rain all summer and the soil is wet.
Neil
Rich, can you make another analyse with RF power of 500 mW with a 10 feet antenna and one with an antenna of let's say 30 feet with 50 mW RF power?? (with the same ground as your first analyse)
There is a statement that says: The antenna is the best amplifier!!
Rob Veld
Response to Rob Veld's request, other system parameters the same as my earlier post:
Tx Power Output (Total Antenna System Length) ... Inverse Distance Field at 1 km
- 500 mW (3m) ... 269 µV/m
- 50 mW (9.14m) ... 258 µV/m
And because some may wonder...
- 500 mW (9.14m) ... 816 µV/m
I knew this was here, luckily I had a very old bookmark saved for this and it was still in place at Part 15 US!
It's a series of articles done by Madmage about medium wave verticle antennas and the comprimises that must be made to get them to radiate efficiently. Some are talking about antenna Q-factor, and it made me think of this article series and try to find it again.
http://www.part15.us/content/chapter-1-classic-vertical
Have a look!
3X the antenna, almost ten times the field strength. I noted from Madmage's article that 5/8 wave is the most efficient vertical. He also mentions, and the chart shows, that field strength falls off rapidly when using a radiator less than .1 wavelength.
If a quarterwave is about 142 feet at 1650, then .1 wavelength is 57 feet, according to my calculations. That could be the reason 50 feet was chosen for TIS total antenna length, something long enough before efficiency took too much of a dive.
Hi, sorry if this is the wrong thread to post this in.
What are your views on broadcasters that go over the legal power limit set by the FCC? I've generally had the opinion that HAM operators hate illegal broadcasters, on the grounds that HAM ops have to get a license and worry that pirates could bring down negative legistlature on their hobby. Do you guys feel the same way?
I am assuming that you mean unlicensed stations operating in the AM and FM bands with more signal strength than is authorized for Part 15 legal unlicensed broadcasting.
If so, then as a ham this doesn't threaten or bother me at all and I leave it up to the FCC to respond. It does not bother me that unlicensed stations use the CB band either. If unlicensed operators migrate to the ham bands then I would be bothered.
I don't see pirate activity as a threat to the status of ham radio operators. The hams are not causing this problem and it is doubtful they will be blamed or be part of a "solution".
As a Part 15 hobbyist this does cause concern because it attracts attention to unlicensed use of the broadcast bands and legal use is often confused with or lumped together with illegal use.
We, at this site, want to promote legal use of the broadcast bands and do not entertain posts advocating or aiding in illegal band use.
Neil
WA4JM The last of the non phony Extra.