[ARC5] Rollerductor setting

[Michael Hanz]

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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