[ARC5] Rollerductor setting

Sat Apr 22 05:54:47 EDT 2017

Good analysis and food for thought.

Very reluctant here to lift a connection and add a turn.

> 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.

Using a Jackson ceramic 150pF variable here (a quality component) with wide
spaced
plates. I figured that high voltage would be present at the transmitter, so
connected the
transmitter to the stationary plates. The low impedance (low voltage) 50
ohm coax inner
(towards the MFJ antenna tuner) is connected to the shaft connection. It
should be safe
to touch:

E^2 = PZ
so for 50W in 50 ohms, E^2 = 50 x 50 = 2500
E = 50V

73 de Neil ZL1ANM

On Sat, Apr 22, 2017 at 7:42 PM, Brian <brianclarke01 at optusnet.com.au>
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.
>
> *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
>
>
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