Word on the street is that a 10' wire antenna has a capacitance of 30 PF at the high end of the AM broadcast band.
At 1700 KHZ, this would amount to a reactance of 3,100 ohms for the 10' wire.
If my math is correct.... To offset this capacitive reactance, a 292 uH coil must be constructed and placed in series with this wire antenna to cancel out this reactance and maximize power transfer between the transmitter and the antenna.
1) Would this coil design be the same for a Talking House Transmitter as it is for the SStran AMT3000 even though they appear to have differing output impedances? (the TH is 50-75 and the SStran is 600 ohms)?
2) Would a properly tuned Base-loaded coil/Antenna be electrically equivalent to a 1/4 way wire antenna (under ideal conditions)?
3) I believe someone (Phil?) mentioned that the Sstran would not allow tuning of a wire antenna longer than 30'. This doesn't make sense to me. If you switched out all the coils in the output stage (no added inductance), shouldn't you be able to match an antenna up to 138'?
You can use 3 100uH chokes from Radio Shack, tied in series to accomplish the goal. Put a variable cap 320pf or so across the series inductors for fine tuning and that should do it.
RFB
"You can use 3 100uH chokes from Radio Shack, tied in series to accomplish the goal. Put a variable cap 320pf or so across the series inductors for fine tuning and that should do it. "
I'm trying to go low-loss here and I think I have everything I need already to build a loading coil. Got plenty of scap white PVC, magnet wire, copper pipe & fittings, old aluminum tent poles and various size electrical conduit.
I'm just wondering if the same base-loaded coil design would work for both the TH (50 ohm) and the Stran (600 ohm)? I would put plenty of extra taps on the coil and probably wind enough turns for 300-350 uH. I would also make the antenna length adjustable to eliminate the need for added capacitance.
Your calculations of the loading coil L agree with my experience. I would not use the small coils due to the loss. Here, I use a coil of #18 enameled wire on a 3 inch form with about 79 turns. Here's a nice design program for coils: http://www.k7mem.com/Electronic_Notebook/inductors/coildsgn.html
At the top of the band a change in frequency of 10 kHz requires a change of two or three turns on the coil to keep resonance. Tuning is very sharp and takes a lot of patience but it is worth the effort. Rather than using taps I use a sliding coil of a few turns to trim the L to resonance. The number of turns on the main coil and the sliding coil need to be determined by experiment. Here's a photo of one of my coils:
The loading coil should work with the AMT-3000 and the impedance match is accomplished by tuning the coil slightly off resonance with the excess L forming part of the Z transformation network. Yow need to follow the instructions in the manual for this.
Question for RFB, your suggestion of using a variable cap "across" (in parallel with) the coil(s) would make that a parallel resonant circuit which is an open at resonance. Should not the cap be wired in series?
Neil
And, would the cap be placed in series with the hot lead between the hot lead and coil or at the junction of the hot lead and coil to ground?
103 turns total on 3" O.D.
Taps at 0, 11, 22, 33, 44, 55, 66, 77, 88 , and 89-103
33 turns per inch
"Question for RFB, your suggestion of using a variable cap "across" (in parallel with) the coil(s) would make that a parallel resonant circuit which is an open at resonance. Should not the cap be wired in series?"
In series I found the loss greater and tuning too wide. The small inductors were used due to wind loading issues of a large box containing a larger loading coil. Out here where winds average 40mph gusting up to 70+..wind loading becomes a very big concern.
RFB
"would the cap be placed in series with the hot lead between the hot lead and coil or at the junction of the hot lead and coil to ground?"
Radiator attached to top end of last inductor. Variable cap connected between 2nd and 3rd inductor and to ground.
RFB
Radiator size was 1/2 inch OD copper pipe with 24 inch cap hat.
RFB
HERE is an excellent loading coil chart I've used several times on both permanent setups and mobile setups.
Note however the chart doesn't list for the middle MW band frequencies. It does list for the lower MW frequencies, either of which is not preferable anyway to the better performance of the upper MW frequencies.
Also to note about the small inductors and reason why I used them. In a case where portability and mobility is desired, compactness is easier to tote around than a bulky box with a large coil, and sits better on a typical music stand tripod (which is what I had on hand). The beefier setup was an experimental setup, later switched the coil out with one made from the linked chart.
Hope that relieves any confusion/conflict/whatever.
RFB