Just out of curiosity, wouldn’t a tuner such as the one made my MFJ Enterprises work to take place of the loading coil of a 3 meter antenna on the AM band? If so i think it would shorten the hours of fine tuning a coil and antenna system. What do you think?
http://www.mfjenterprises.com/Product.php?productid=MFJ-925
Tuner
My answer is a “definite maybe”. Such a tuner would need to tune the BCB and be able to work with an impedance of 3000 to 4000 ohms. There is also a question as to whether the tuner input impedance when tuned will be acceptable to the particular transmitter in use.
Probably the only way to tell for certain is to try it but I don’t hold much hope for it to work.
Neil
Well, the transmitter output would have to be close to 50 ohms?
Is that right? I have a little MFJ tuner which
will handle 100 watts on the 160 through 10 meter
ham bands. It wants to see 50 ohms on the input.
Most Part 15 transmitters aren’t like that. (Although
the Ramsey AM-25 output is in the 25 ohm range,
I think.)
I wonder if you could use the MFJ tuner to “fine tune”
a Part 15 3 meter antenna and loading coil system.
In other words, the loading coil that is usually used
would be in there, and the MFJ tuner would fine the
loading coil.
I don’t know. The lading coil might have to have
different characteristics.
I have actually thought about this myself in the
past. But it might be pretty complicated.
And Neil is right. The tuner would have to tune
the AM BCB. It’s might do 1700 kHz as is, MAYBE.
But it probably wouldn’t go much below there, even
if it did.
Bruce, The Dog Radio Group
Transmitter Source Impedance vs. Load Impedance
The internal source impedance of a transmitter (including a “Part 15” transmitter) almost never is specified, or probably even known accurately by the equipment supplier.
But for maximum transfer of r-f power to the load impedance (antenna system), the transmitter source impedance should be as low as possible — and hopefully not more than a few ohms.
If it equaled the load impedance then half of the output power capability of the transmitter would be lost inside the transmitter as heat, because the r-f power produced by the transmitter divides and is dissipated by the ratio of the source and load impedances.
The load (antenna system) impedance at resonance of a Part 15 AM transmitter can vary from a minimum of maybe 20 ohms to 100 ohms or more, depending on the r-f losses in the loading coil and connection to r-f ground.
The published specifications of most commercial transmitters are defined for the load impedance that they are designed to drive.
Loading Question…
If the MFJ tuner will tune low enough, you should not need the extra loading coil unless as Neil stated the tuner will not work with 3000 to 4000 ohms antenna impedance, that of a 3 meter wire antenna at that frequency.
The 160 meter Ham band starts at 1800 kHz. If you are operating at the very top end of the AM band, it might work. If not then I would try adding a small amount of inductance (loading coil) to the antenna.
Antenna System Impedance Matching
Just to note that there is more to impedance matching than using only a loading coil to offset the capacitive reactance of an electrically short antenna system.
For a given d-c input power flowing through the output electrode of the final r-f stage, any transmitter including a Part 15 AM transmitter delivers its maximum r-f output power only into the non-reactive load impedance for which the transmitter was designed.
For examples, if that transmitter design assumed a load impedance of 50 +j 0 ohms,* and the actual load impedance is either 25 +j 0 ohms or 100 +j 0 ohms, then the load SWR for both of those load impedances is 2:1.
This means that 1/3 of the available r-f energy is reflected from the antenna system back to the transmitter, where most of that reflected energy is dissipated as heat.
* The “j” term shows the reactance present in the load, including the effect of the loading coil. Zero reactance shows that the load is resonant.
Specific answer to OP
rock95seven:
That particular tuner is an automatic tuner and seems like an ideal solution. However, the manual shows that it is an “L-net” tuner with inductance and capacitance ranges way wrong for a short 3 meter antenna.
In particular, the maximum inductance is only 24 uH. At 1600 kHz, the capacitance required to resonate with 24 uH would be about 400 pF. Since a 3 meter antenna only has about 30 pF capacitance, the tuner would need to parallel about 370 pF to get resonance. Since it apparently can go up to 3900 pF, that’s not a problem. The real problem is paralleling 370 pF across the 30 pF antenna will shunt a huge amount of current away from the antenna, effectively killing the range.
A possibly better solution to investigate would be the MFJ-16010 Random Wire tuner, http://www.mfjenterprises.com/Product.php?productid=MFJ-16010. It’s not automatic, but it promises to have more suitable-valued components. Trouble is, the manual shows the capacitance range, but not the inductance range, so whether or not it will tune efficiently to the upper AM band is a total crap shoot. Maybe MFJ will respond to an email requesting the inductance range and toroid core material specifications?
To OP, cont’d
The real problem is paralleling 370 pF across the 30 pF antenna will shunt a huge amount of current away from the antenna, effectively killing the range.
Even if the coil in the MFJ-16010 Random Wire Tuner could resonate a 3-m antenna system in the AM broadcast band, its schematic shows that the minimum capacitance to ground across the r-f output line is 10 pF.
So a shunting loss will occur there, also (unless that capacitance is removed from the circuit).
Another factor could be that the taps along the coil in the MFJ-16010 may not have enough resolution to produce the ~exact inductive reactance needed to resonate a Part 15 AM antenna system.
It would be worth a try, especially if it can be returned to the vendor if it doesn’t prove satisfactory.