RE: NEC-LIST:Loaded dipoles

<tuli_at_bellatlantic.net> wrote:
>The antenna is multiband (40,80,120,160 MHz + 20 MHz, +/- 2MHz around each).
>The idea is to feed the 160MHz monopole and get the match by proximity
>coupling for all other monopoles. I'm all set for all other bands except the
>20 MHz.
>A matching network at the input will affect all the other bands (lower
>efficiency) which I am trying to avoid.
>This is why The only way (I see) is to wrap the 20 MHz dipole in a resistive
>film. This way, only the 20 MHz monopole will be affected (see "The
>Open-Sleeve Antenna" by Roger A. Cox on CQ, August 19983 pp.13-19).
>The inductive loading would be by replacing the monopole with a coil ?

Resistive loading of the antenna accomplishes nothing but to
dissipate power. If dissipation is your goal, you can get it more
easily with a resistive pad in the feedline between the antenna and
your transmitter and/or receiver. The only advantage of distributing
the resistance that I can imagine is to dissipate the heat into the
air more easily.

To test my statements, use NEC. NEC can distribute the loading, as
well as lump it.

You are correct that "a matching network at the input will affect all
the other bands." However, efficiency defined as power radiated
divided by power input is not affected, except trivially by resistive
loss in the matching-network components.

The frequency range you give is 8:1. I have a very similar situation
myself, in my ham station. I operate all ham bands 80 through 10
meters, i.e., 3.5 MHz through 28 MHz. Here's what works for me. It
gives me a VSWR of 6 or 7:1 in the worst cases (7 and 14 MHz), and
under 1.7:1 on 3.5, 21, 25, and 28 MHz, excepting one pathological
special case of 17:1 at 10 MHz, which doesn't bother me because hams
aren't allowed to transmit more than 200 watts at that frequency,
whereas I can and do transmit 1.5 kW on all other bands. Since this
pathological case is at one-third of my highest frequency, it seems
equally irrelevant for you.

My antenna is a horizontal wire 30 m long, fed at its center with a
parallel-wire transmission line having Zo = 600 ohms, and 12 m long.
Then there's a 4:1 balun transformer, then an unbalanced C-L-C "tee"
matching network to a 50-ohm coaxial line to my transmitter and
receiver. In other words, the matching network comprises a series C,
then a shunt L to ground, then a series C. This is a "high-pass"
filter configuration. It is fixed-tuned to give me a perfect match
to 50 ohms resistive at 3.7 MHz. I.e., I never adjust it.

You could easily adapt my scheme to your situation by making it
unbalanced, since you say you have a monopole, not a dipole; and by
scaling it in frequency. My scheme is essentially lossless, as it
must be because I transmit 1500 watts.

73 de Chuck, W1HIS

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