Several excellent responses to my question about the need for preamps on
Beverages were received. Here's the summary:
Carl, KM1H:
Good question Larry. A properly transformered Beverage should not require
a preamp. I really do not understand why some insist on using them.
Loops, EWE's and other low efficiency antennas do require a preamp.
What could be very helpful is a hi Q passive and tuneable preselector.
Several years ago I measured the NF of several receivers; R4C, TS-830,
930, 940, IC-735, 761, 765, Paragon and a few others. At 14 and 28 MHz
the NF on all were in the 9 to 13dB range.
I can not speak for the latest crop of rigs with switchable preamps, etc.
but a part of that NF is in the front end filters and switching diodes.
Perhaps someone has run tests on the TS-870, FT-1000D and MP, etc.
Rob, K2WI:
I have never found it necessary to use a preamp on a Beverage, from the
standpoint of noise figure. I have always heard the noise level go up when
attaching the Beverage to the receiver. Many times, though, the signals
still sound weak, masked by hiss from the audio stage of the receiver. I
think that the perceived benefit of using a preamp comes from increased
system gain, not from enhanced noise figure.
The situations in which I have found a preamp useful have usually involved
splitting a Beverage to feed multiple receivers, or with certain receivers
like the TR7 and R4c which seem to lack overall system gain.
John, ON4UN:
In my book I say, for almost 10 years, that a preamp is only necessary
unless you have a very long and lossy feed line and want to compensate for
that. All the rest is BS. But some people like to sea an S meter move.
Guess they hear with their eyes, I listen and hear with me ears, hi..
Simplified: when the noise produced by the antenna is still above the
internal noise (hiss, whaever you call it) of you RX, you are fine.
It is so simple..
Tom, N4KG:
I have no preamps on any of my 7 Beverages.
With a matching transformer, I find that they still
hear atmospheric noise so there is no point in
adding more gain. WITHOUT a matching transformer,
I expect a preamp would be necessary to overcome
the mismatch loss (approx 10 dB).
de Tom N4KG
Willy, W3GCG:
What type of beverage impedence matching tranformer are you using ? Sounds
like an auto tranformer. Try winding one like I make for all the DXers and
contesters on the low bands. I wind 16 quadfiliar turns on a ferrite toroid
with a mu of abt 125. I use FT-114-61 cores and mount them in a black plastic
box with SS hardware. I have been making them for abt 17 years and never have
recd a complaint !! 73, Willy Usually no preamp needed!!!
Yuri, K3BU, VE3BMV:
Preamps can be usefull on 160 & with Beverages in number of situations:
1. Preamps with tunable input help to eliminate out of band QRM, like BC
stations.
2. If you use rig like TS870 that can switch Beverages and TX antenna (with
famous K3BU diode mod.), preamp can bring up Beverage signals to the level of
TX antenna, then with attenuators and AIP it is possible to level the signals
so they sound more less same strength. Saves goofing with controls when quick
switch/check needed. (Similar effect can be had by using attenuator plugged
between the TX antenna on receive and RX ant. switching) The best results are
usually with RX gain set at the point when the band noise is about starting to
move Smeter.
3. When Beverages (without preamp) "die" with sunrise (almost no signals) at
VE1ZZ, preamp brings them back to life, and quite often they beat 4 square in
hearing the weak one.
4. It is good to have the preamp, in case you need it. Another option in QRM -
weak signal digging arsenal.
5. Can be used with weird antennas like 40 m beam, TV antenna, Dog Loop, etc.
to bring the signal up when all other antennas don't.
I picked up Palomar Engrs. preamp at the fleamarket, and is the the #1 used
here. It appears to have good high Q tuned circuit and smoooth gain control.
But I want to build my own, with multiple hi-Q tuned stages. Also looking for
that high gain/low noise super device and super circuit.
Bob, VE7BS:
Not everyone Larry.
At the request of WorldRadio editor, I recently wrote an article explaining
why I never run the RF gain control (actually an IF gain control) on the
Corsair 2 anywhere near full setting.
But some receivers are designed on the assumption that there will be local
noise on 160. The OMNI-C for example has a sensitivity of 2uV on 160, and
when I was using it a few years ago with a Beverage I had to use a preamp
out here in the quiet boondocks.
Ink, N4OO:
I have never seen a need to use a pre amp on anything except the small
receiving loops. Signals on the Beverages are typically 10db or so down from
the L transmitting antenna. Likewise for the EWE antennas that I tried and
discarded (no F/B). Even short Beverages, on the order of 300 ft, deliver
plenty of signal here.
Summary:
The question was asked because we are helping build an antenna farm at
W9EFL, who has plenty of room for beverages, active arrays, or whatever.
Here at N9DX, unfortunately there is not enough room for Beverages, unless
I use Yuri's "Midnight Beverages."
The consensus is obviously that there is no need for a preamp to establish
noise figure, because the receiver noise figure is already low enough in
most cases. However, Yuri makes some good points on considerations other
than noise figure. And Tom, W8JI, pointed out recently that the preamp
need not be outdoors.
Thanks to all for the many thoughts on the subject.
73
Larry, N9DX
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|