David DeFord wrote:
> Putting toroids on the cable is never detrimental to the performance of
> the antenna. In a few cases, they will make no difference at all, such
> as with ground plane antennas mounted on metal airplanes. Their effect
> on the radiation pattern is typically not large, but the net effect
> should virtually always be beneficial, especially in the case of a
> vertical antenna fed by a horizontal run of coax.
>
> Of greater importance to us are the other effects of current flowing on
> the outside of the coax shield, which is what the toroids suppress.
> This current can couple into nearby wires and electronic devices,
> increasing the potential for interference from the radio transmitter. I
> would rather keep the radiation confined as much as possible to the
> antenna in the tail, rather than allowing it along the length of the
> feedline.
>
> Current on the outside of the shield can also affect the reading of an
> swr meter, making it more difficult to find the dip that occurs when the
> antenna is trimmed to the resonant length. For me, there is no doubt
> that the benefits of the toroids are well worth the dollar cost and few
> grams of weight.
I'll second all the above, but I looked it up and Archer says his
antennas don't require a balun because he uses "modified version of a
feed called a Gamma match that feeds the antenna at the fifty ohm
point." But there's an "f" - frequency - in the formula to do Gamma
matching and there still may be some mismatch in a broadband antenna
like we have here. So one could still have some current on the
outside of the coax on "off-peak" frequencies.
One should never route antenna coax in proximity to any wiring, but
unfortunately in a plastic airplane that wiring will always be
affected by the much larger amount of radiation from the antenna
itself. The beads need to be of the correct type to work, and on a
homebrew copper tape antenna they do no harm, although the impedance
mismatch is less than potentially on Archer's (looks like) folded
dipole.
It's also true - a factor in the original post - that the beads do not
have to be placed at the antenna feed point. If necessary to try and
cure an interference problem, they can be placed anywhere. It
logically follows that the place to start is 1/2 wavelength away - 41"
- and see what happens sliding them in either direction. An
alternative for someone not desirous of trying to locate Type 43, 120
mHz ferrite cores with proper size I.D. where there may be no problem
to solve, or a problem that ferrites won't solve.
Best,
Fred F.
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