G'day All,
Here is my two cents worth...
I think that there is not a great deal to be gained from using
multiple-tuned verticals. The reason I think that is that if you look at
the aim of improving our short vertical antennas, we see that we are trying
to improve the ratio of radiated power versus applied power. We tend to
concentrate on radiation resistance alone, when it is the ratio of
radiation resistance to losses which determines how much goes into ether
and how much warms up the local environment.
Radiation resistance is an entirely ficticious resistance which is
evaluated by analysing the impedance at the feed point. Change the
feedpoint and you can change the radiation resistance. The notion that
there is some inherent radiation resistance for a particular antenna
configuration (say 75 ohm for a 1/2 wave dipole) is a mathematical one and
arises out of the practice that by convention and convenience the analysis
is carried out using a feedpoint where the current is maximum. Radiation
resistance is then actually the lowest value of the radiating part of the
feedpoint impedance possible. The change in radiation resistance with
feedpoint is viewed by some as simply a transformer action. A good
explanation except you can not 'transform' the radiation resistance of a
given radiating structure to a lower value, only higher. However,
transformer action remains a good way of viewing the situation.
Where does this leave the multiple-tuned vertical? Well, I think that
indeed the radiation resistance will be transformed up as a ratio N * N,
and you will need larger coils to resonate each leg, and so on. The fly
in the ointment as far as I can see is that while the currents in each leg
are reduced, any losses (earth, coil, etc) as seen from the feedpoint are
also multiplied up by N * N. So my guess is what you gain on the
roundabout (increased apparent radiation resistance) is lost on the swings
(increased apparent losses). That is, the ratio of radiation resistance
to total losses remains largely the same.
So why do commercial installations use the configuration? I suspect it has
more to do with the GIGANTIC powers they use and the difficulty of making a
single earth connection carry all the current at those powers. By using
the multiple-tuned vertical the currents are split and so are reduced at
each individual earth point.
The above has been seen to be the case using antenna analysis software.
Initially I had a great rush of excitement when I saw a 0.01 ohm radiation
resistance increased to 1 ohm using ten radiators, but this quickly
subsided when I introduced a series loss resistance with each coil and saw
that that loss was multiplied by the same factor :-(
In conclusion I feel that there might be something to be gained (not a
great deal), but also there may be some other factor not accounted for
which may produce better results than expected from the above. After all,
we are experimenters and are not totally bound by theory, but only
imagination :-)
73s Steve Olney (VK2ZTO/AXSO - QF56IK : Lat -33 34 07, Long +150 44 40)
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LowFer URL:
http://www.zeta.org.au/~ollaneg/lowfer.htm
AXSO Experimental Station URL:
http://www.zeta.org.au/~ollaneg/axsoextx.htm
LF Receiving - FRG-100
LF Transmitting - 177.5kHz 8W to 7.6m top-loaded vertical
Modes - AM, SSB, PSK31, SSTV, Hellschreiber and QRSS
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