Hi Earl and all,
> Puzzled why the steel wire so noticeably outperformed the copper, I
> thought about this a bit and came up with what I believe is the reason. A
> Beverage is actually a bidirectional antenna, however we all try to
> attenuate the signal coming from the unwanted direction by terminating the
> wire with a resistance to ground at one end to absorb the unwanted signal.
> This is what gives the Beverage such a good F/B ratio and therefore a
> good S/N ratio. But not all of the signal from the unwanted direction is
> absorbed by the termination resistor -- some of it is reflected back down
> the wire toward the receiver, hence limiting the F/B ration and the S/N of
> the antenna.
I've found the same in some cases, and the opposite in others.
If the Beverage is properly terminated, then the steel wire works no
better and maybe slightly poorer than copper wire. Also, a 2 wl
Beverage is not any better than a one WL Beverage.
If the antenna is misterminated, the additional loss of steel wire
seems to help.
In practice, I think I have seen a slight advantage to copper wire
when properly terminated. I believe the key to the differences is
mostly in proper termination.
If the antenna is properly terminated, attenuation along the wire will
only hurt directivity. It will not improve it. It is important that the
entire antenna carry uniform current, otherwise additional wire
length added to the far end will not change directivity, and if
properly terminated additional wire loss can NOT improve F/B. .
If the antenna is misterminated, additional attenuation in the wire
will "self-terminate" the antenna by adding distributed loss, and
make it work better.
This weekend, I added a new group of Beverages and measured
the termination using two four-five foot copper pipe rods driven in
our very dry (we have been in a drought) soil. It was impossible to
get a perfect termination, and the antenna had standing waves. The
termination resistor wound up being 250 ohms, but still never was
stable with frequency change.
I pulled in two 1/4 wl buried radials, and the best termination
resistance moved up to 460 ohms and the feedpoint was almost
perfectly flat up past 5 MHz.
I always check the termination by sweeping the antenna (fed
through a GOOD matching transformer) and looking for ripple in
SWR. I try to obtain a constant SWR as frequency is varied. When
properly terminated, the VSWR stayed at 1.45-1.55 : 1 over that
entire range (75 ohm transformer measured on 50 ohm meter).
The velocity of propagation was about .9. 475 feet of wire eight feet
high looked like a full wave.
When I measure current after adjusting that way, it has a smooth
uniform taper all the way to the far end. In about 500 feet, about
70% of the current is left. That means the copper antenna has
about 3 dB attenuation all by itself, mostly due to ground loss. I
believe that attenuation is why I don't see the expected change
when properly terminated antennas are made 2 wl long. I DO see
an improvement in performance when I slightly misterminate the
antenna, and then make it longer. But it is never any better than a
properly terminated 1 wl antenna, at least not here.
I expect over better soil, or even perhaps much worse soil, the
losses would be lower and a longer antenna could work better. But
there is little doubt that adding additional distributed loss can only
hurt a properly terminated antenna.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|