TopBand: Folded unipoles

[Peter Chadwick]

Lane, KM3G, asked about these some time back - I've been on vacation for
two weeks, missing the K7K, which is one of my last 8 wanted countries.
Grr!

I use a folded unipole on 160, and it works - so much so that I'm an
alligator. I have a crank up tower with a 5 ele monobander for 20 at 62
feet and 4 ele monobander for 15 interlaced with a 4 ele monobander for
10 at 68 feet. I run three #16 wires spaced about 10 inches apart by
copper pipe spacers up to the top of the tower, as a feeder. All the
coaxes to the beams are bonded to the tower at the top, and run down the
opposite side of the tower to the three wire feed. They are all bonded
at the base of the tower back to the tower, and I use three four foot
ground rods spaced round the tower. No radials - when I tried them,
there was no measurable current in them on a 250mA thermoammeter. The
soil is thick heavy clay. The tower is fed by an L network, with about
10 microhenries and 700pF for 160, which covers all the band with SWR
<2:1. On 80, there's an L network of 20microhenries and a 100pF variable
- the Q is high on 80, and the voltage large (abt 2800v).

The arguments about folding raising radiation resistance or not are
interesting. If it doesn't alter the radiation resistance, what is the
radiation resistance (as opposed to feed impedance) of a folded dipole?
75 ohms?  It's interesting that Lamont V. Blake in his book 'Antennas'
(Wiley, 1966) says that 'If  there is no ohmic loss in the antenna -
that is, if all the input power is radiated - then the radiation
resistance referred to the feed point is equal to the resistive
component of the antenna input impedance'. A similar  statement appears
in Glazier and Lamont's 1958 ' Transmission and Propagation' - the UK
armed services handbook Vol 5. However, there was a general disagreement
on this reflector a few months ago that folding the radiator raised the
radiation resistance. All I can say is mine works! as N6TR knows - he
heard me a couple of weeks ago before I went off on holiday.

Certainly raising the feed impedance can help reduce matching network
losses by reducing the large currents.

One point is that I use a T2X rotator, and burnt out the direction
indicating pot. I can only surmise the RF current did that, so I have
bypassed the the ends of the pot to the wiper with 0.01 disc ceramics. I
also have a toroidal choke in the feed to the rotator up at the top, and
another at the bottom, and a heavy braid strap from the head nit of the
tower to the stub mast to keep currents from flowing through the
rotator. From experience, when bypassing the rotator cables, use big
enough capacitors - I used 50vdc disc ceramics at 0.1 mFd, and had them
catch fire from the 28volt 50Hz.

Try the folded unipole Lane - you may well be surprised. But you need a
low noise rx antenna - I use a tuned loop, with a push pull cathode
follower using a 6DJ8. It stands up to the signals induced on the loop
when on tx better than these solid state horrors! It should be more
robust against static as well. Hasn't failed in three years anyhow.

73

Peter G3RZP
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