TopBand: Re: Slow wave antennas

[W8JITom@aol.com]

Carl,
This really isn't an antenna forum so this'll be my last on this.....

In a message dated 97-01-15 21:48:56 EST, you write:

>One does not introduce lumped reactances into a transmission line without
>a full understanding of the consequences and the means to measure and
>control them. 

That was my point. A slinky antenna depends on correct phase and current
distribution, just as any antenna does. Cramming 1000 ft of wire into a 200
ft space only helps all the time if you are making a low frequency AC or DC
resistor. In an antenna, phase and distribution of current are paramount.

>Next, by placing lumped inductances in the wire at selected intervals
>would seem to limit the antennas usefullness to a single band or maybe
>selected harmonics. 

Delaying wave velocity a substantial amount reduces useful bandwidth anyway,
once you exceed a certain ratio of L/C. There is little difference between
adding phase retarding inductance at distributed points less than 1/4 wl
apart and a continuously distributed inductor. Besides, I'll take every dB I
can get on 160. 

Did you determine the optimum pitch per length of the slinky, or is this left
to chance? Once optimum phase shift is approached, things change fast. As
little as 45 degrees change in phase shift can make a large difference in
directive gain, while a 180 degree error can be devistating. Accidentally
stumbling on the optimum value seems unlikely.
   
>I still have a very hard time understanding how
>periodically slowing the VP and then speeding up will result in a good
>performing antenna.

Me too, that's why I never add capacitors in series with the antenna after
the coils...or use too much inductance (slowing vp down too much is the same
as speeding it up, or worse).

>.Im trying to relate this to the incoming wavefront
>which is a constant. 

It's all phase shift. The POOREST way to obtain directivity is to construct a
constructive phase end-fire array. Phase addition slowly builds up signals
and directivity with length. The BEST way to get directivity in a small space
is to null unwanted directions by destructive phase addition in undesired
directions.

Examples? Compare the directive gain of a 1/8 wl spaced pair of elements fed
out-of- phase to gain of a uniform phase broadside collinear array the same
physical size. 
Compare a 1/8 wl end-fire couplet to a long wire antenna. The phased elements
have ~4.8 dB directivity in a 1/8 wl linear area (the antennas can be small
loops for receiving) VS the same directive gain from a 6 wavelength longwire
(if current is uniform in the longwire, something impossible to do).

If the same two close spaced elements are phased for maximum signal addition
(constructive phasing) only gain will be greatly reduced. Trying to match
array distributed phase with incoming signal phase usually just wastes space.

For a destructive phase system in a short "long wire", model two in line
terminated "longwires" 125 feet long (like mini-Beverages). Feed the back
(away from termination ends) antenna with a 180 degree phase reversal
transformer through a 70 foot electrical delay line, the front antenna (one
closest to terminated ends) is zero degrees reference. (For El-nec users, the
160 meter phase delay would be +135 degrees on the element in the null
direction and zero on the element nearest the termination ends). You can't
match that pattern with a dozen in line slinktennas, and with the cross fire
delay line phasing and proper termination it works from below the broadcast
band to upper HF.

73 Tom

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