[Fwd: LF: LF antennas]
Fri, 27 Feb 1998 23:38:19 -0500
Peter Dodd wrote:
> >From Peter Dodd, G3LDO
> Horizontal versus Vertical antennas
> The e-mail from G4GVC, G3XDV and G3JKV on antennas came at rather a
> crucial time.
> I am in the process of finishing The LF experimenter's Source book
> and was considering the inclusion of a very good article from the
> Lowdown Magazine (Publication of the Longwave Club of America). This
> article , called The Texas Beacon by Bill Cantrell gives an excellent
> description on the design and modelling of a vertical antenna with a
> large capacitive top hat. The problem was that it runs to 16 pages,
> which would take up a large chunk of the chapter, concentrating on
> the conventional professional approach to LF antenna design.
> After reading the E-mail I felt that the space would be better used
> by quoting the e-mail and looking for other avenues for antenna
> experimental work. I have researched the latest EZNEC antenna
> modelling software from and, in spite of some initial reservations, I
> feel that it is a useful tool for modelling antennas at 136kHz.
> Before I erected my rather unconventional antenna (already described
> in a previous e-mail) I modelled it using EZNEC 2 and found, to my
> surprise, that the polar diagram was nearly the same as a
> conventional Marconi. I didnt know if this was due to the limitations
> of EZNEC 2 but I did not feel discouraged and I constructed the
> antenna. In the event it worked fairly well.
> To make DX contacts with the power available to us we must use
> skywaves. The traditional Marconi has a very deep null at high
> radiation angles and it is difficult to find an antenna, fed against
> ground that will fill in this null and it is difficult to get any
> horizontal polarisation at all if you are feeding the antenna against
> ground. This is not surprising. If you consider the DDRR antenna, on
> a quarter wave 14MHz element only 200mm of vertical element does most
> of the radiating.
> The best practical antenna I have found in the very short time I have
> had for experimenting, using the EZNEC2 model, is a 100m long wire
> antenna at zero feet one end and 14m high at the other. It is fed
> against ground with appropriate inductive loading. So you might be on
> the right track John (G4GVC).
> I think we have to be careful not to get mixed up with vertical and
> horizontal polarisation and the vertical angle of radiation even
> though one often goes with the other.
> You can get a very high angle of radiation by using a loaded
> horizontal dipole so we might get some interesting results from
> GW3XDV in the near future
> The discussion on the use of EZNEC2 modelling will be accompanied
> with some interesting pictures in the second edition of the The LF
> experimenter's Source book
> Peter Dodd, firstname.lastname@example.org