Dear Richard, LF Group,
The umbrella antenna has a capacitance of about 144pF (against 129pF
calculated by EZNEC) and a resistance of about 70ohms (rising to nearer 80
ohms after some rain). It was tuned by a tapped series loading coil with
variometer for fine tuning - the total inductance required is around 700uH.
The coil added a further loss resistance of 9 ohms. Resistance matching to
the TX was done with a multi-tapped auto transformer wound on a small SMPS
core. To prevent a lot of RF current flowing down the coax feeder to mains
earth and bypassing the radials, I used a common-mode choke in the feeder at
the ATU end consisting of about 20 turns of RG58 coax on a large ferrite
toroid core.
I also tried the idea of tuning the capacitive reactance of the radials
using a coil in series with the ground connection to the ATU. This used a
small tapped coil of about 50uH maximum between the ground terminal of the
transformer, and the common point of the 4 radial wires. I measured the RF
voltage between the transformer ground terminal and a "real ground" ground
spike at the bottom of the antenna using a diode voltmeter. With no radial
tuning inductance, the RF voltage was about 90V with 1A antenna current.
Adjusting the tap on the coil gave a minimum of 17V (including a few V due
to the local broadcast stations!). This required about 26uH of inductance -
the main loading coil has to be reduced by the same amount to maintain
resonance. It is not possible to get 0V in this way, because the tuning
inductor cannot tune out the resistive component of the ground impedance,
and anyway, the "real" ground potential is a bit arbitrary, but clearly a
substantial reduction is possible, which would be useful for QRO operation.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Richard Newstead" <[email protected]>
To: <[email protected]>
Sent: Wednesday, June 10, 2009 7:58 AM
Subject: Re: LF: Re: M0BMU small antenna test beacon
Hi Jim
I was out so I missed the tests last night. What ATU configuration did you
use? My system is just awaiting a suitable day for initial tests.
73
Richard
G3CWI
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