[ARC5] Rollerductor setting

[Brian]

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.

On Friday, April 21, 2017 10:53 AM, you said:

As posted in a related thread I run an air variable cap in series with an UNUN for my SCR-274N setup and just a series cap for my T-19/T-22 setup.  Both work fine.  I get around 35 to 40W out on 40M and a bit more on 80M 

73 Mark K3MSB

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20170422/0cd3d066/attachment.html>