[Elecraft] Portable Antennas

[Erik N Basilier]

Ron, I agree on all points.

My first antenna was a 40 m dipole with vertical end sections at about 35 
ft. Initially I fed it with 300 ribbon cable. Although the antenna was cut 
roughly for 40m it was probably a bit off. The SWR would have been a minimum 
of roughly 4:1 but with the vertical ends and moderate height I would guess 
6:1 or more. I used a home made balanced tuner and an incandecent bulb to 
tune for max current. This setup worked, but it worked much better later 
after I replaced the ribbon cable with home made ladder line. I guess 
because of this experience I have never again bothered to try 300 ribbon 
cable in the field. I have used window line in the field, but only for car 
camping; I consider it too unwieldy for backpacking. For a 1/4 wavelength 
dipole, in a choice between good high-Q loading coils vs window line, I'll 
take the loading coils most of the time. However, the very best choice, that 
I haven't yet tried, may be widely and haphazardly spaced single wires for 
the feeder.

73,
Erik K7TV

> Quite true according to the literature and the antenna simulator programs
> (e.g. EZNEC).
>
> Since height of a horizontal radiator has a huge impact on the ground
> reflection gain, it's not uncommon to find a substantial *increase* in
> signal strength for short-skip operation by shifting to a lower frequency
> band where the antenna is only 1/4 wavelength long.
>
> Actually, such an antenna does not need to use loading coils. The 
> difference
> in gain between a center fed radiator 1/2 wavelength long and one only 1/4
> wavelength long is less than 0.1 dB.
>
> Where more losses usually occur is in the feed line and matching network,
> wherever it exists. "Loading" the elements or putting some other sort of
> matching network at the antenna helps provide a low SWR for efficient
> transmission line operation, but if a low-loss line is used the matching 
> can
> be done at the rig just as well.
>
> In all cases the losses are almost entirely the resistive losses in the
> conductors, typically the "loading coils" when they're used or in the feed
> line if a high SWR exists. Remember, we're not talking about d-c 
> resistance,
> but RF resistance which is typically much, much greater since 'skin 
> effect'
> forces all the RF current into the surface of the conductor. Small 
> diameter
> conductors have far more loss at RF than their d-c resistance might 
> suggest.
> So it's important that largest possible wire in the smallest possible coil
> be used for loading. That's obviously a big trade-off, of course, one that
> Buddipole seems to have handled well.
>
> Ron AC7AC