[ARC5] Rollerductor setting

Sat Apr 22 10:55:28 EDT 2017

I , too, am reluctant to add turns or modify the transmitters. These poor things have been hacked enough over the years!
Here is an observation that I 've made with the series cap and unun setup:While the watt meter on the 50 ohm line shows 50 watts the BC-441 ammeter looks to be much  less than 1 amp!This does not square with a straight 12.5 ohm match which should be just under 2 amps.(I get 2 amps when I run it with a 10 ohm load in series with a 100 pf cap)

Perhaps adding the series cap and peaking the output changes the nature of the tank circuit into a dual tuned resonant transformer.In this scenario, the VOLTAGE in the secondary is mostly ruled by the ratio of primary to secondary, critical coupling, and Q.( see Terman "radio engineer's Handbook" page 155).
So this means that there is a voltage rise and current reduction in the secondary. Now the readings make more sense.
Say the Xc of the series cap  is 400 ohms and the current is .5 a , that's  only 200 volts peak.
The "centering" of the roller inductor has been useful to trim the antenna match, as it will be more capacitive on the low end, needed more inductance and vice-versa.
So far so good. I've made QSOs up and down the east coast without really trying too hard and 599 signal reports ( for what that's worth).Even the AM voice has gotten good reviews as 800 miles away. (It's harder to find AM'ers).

 On Saturday, April 22, 2017 8:11 AM, Michael Hanz <aaf-radio-1 at aafradio.org> wrote:

  All good stuff.  It may also be useful to remember that things can go south in a hurry when changing frequencies, depending on how close to a quarter wave your antenna is.  See the impedance curves in Chart 2 at http://aafradio.org/docs/Aircraft_Antenna_Design.html for example.  I'm not familiar with the MFJ tuner, but the active tuners that were used for HF in postwar military aircraft (AN/ARC-21, etc.) had only one job to do, and that was to condition the antenna to represent a purely resistive load for the transmission line at the operating frequency.  That made tuning the transmitter much less complicated.  Perhaps the MFJ doesn't have the parameters necessary, but I think I would be considering ways to modify *it* rather than the command transmitter.  That way you ought to be able to run a 4:1 or 9:1 unun by itself between the tuner and the command transmitter.

           73,
  - Mike  KC4TOS

 On 4/22/2017 3:42 AM, Brian wrote:

   OK folks,   Time to get back to basic principles.   A pair of 1625s in parallel probably requires a resistive load of about 1.2 k Ohm. The tank coil has about 20 turns – the lower frequency sets have more turns and the 7 – 9.1 MHz set has fewer. The variable coupling coil has 4 turns. So, the turns ratio is about 5:1. Therefore, the impedance ratio is about 25:1. Hence, the output resistance required, if we follow the Jacobi theorem, should be 1.2 k / 25 = 48 Ohm. By altering the degree of coupling using the ANT COUPLING knob, matching down to 5 Ohm should be easy. But the coupling coil doesn’t reach full coupling, unless you open the transmitter  top cover and reset the coupling gear on the coupling coil shaft; so, 48 Ohm matching is not possible. The simpler way is to lift the grounded end of the coupling wiring and put that end in series adding with a turn or two of insulated wire wound round the bottom end of the tank coil. An extra turn gets you an impedance ratio at full coupling of about 16, thus giving an output resistance of 75 Ohm, variable by changing coupling. With extra turns, the lowest antenna resistance you can match will increase. So, now you can see that resistance matching was done with varying the coupling.   An average HF antenna on an aircraft would stand off the fuselage say about a metre, possibly be 3 mm diameter and might be 10 m long. The radiation resistance would be 1 to 2 Ohm, depending on frequency. The capacitance between the antenna wire and the fuselage would be about 256 pF. You can use the roller inductor to cancel out this capacitance. At 7.15 MHz, you will require about 2 uH. However, if the coupling range is not sufficient to match the resistive component, then you can use a capacitor between the ANT terminal and antenna feed-point. The original supplied with the BC-442 or the RE-2 was 75 pF. What you now have is a capacitive divider. The roller inductor now needs to cancel out a smaller capacitance – two capacitors in series. So,  you will need more inductance. The notion that you start with half or half of half of the roller inductance is relatively meaningless unless you know the specific radiation resistance and capacitance.   Now, say your home antenna is also 3 mm diameter wire, but 10 m above ground and 15 m long. The capacitance will now be about 284 pF. And the radiation resistance will be higher. At the higher frequency of the 40 m set, if your antenna is 0.18 of a wavelength above ground (7.2 m), its radiation resistance will be about 50 Ohm, and its capacitance will be 295 pf. The likelihood of matching to an original 40 m Command Tx is close to zero. So, adding a turn to the coupling system and possibly using a series capacitor should get you there. Using a UNUN is irrelevant and may introduce losses.   Say you claim to get 40 W into a 50 Ohm antenna from one of these rigs. The Voltage at the input to the antenna will be about 45 V. Not very bitey. But if you use a series capacitor to achieve your impedance match, the Voltage at the rig’s antenna terminal can be 450 V if your impedance transformation is 10:1. The average broadcast band tuning capacitor may flash over. I wouldn’t recommend varying the capacitor with a bare hand. You can also see that with an aircraft’s much lower radiation resistance, the Voltage at the Tx antenna post would be much, much higher – which is why a 10 kV vacuum capacitor was supplied.   73 de Brian, VK2GCE.

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