[Lowfer] active whips

[JD]

>>> I was always very skeptical of active antennas . My engineering 
>>> background made it hard to believe that less conductor could be better.

More conductor generally means the ability to extract more energy, but it's 
safe to say that doesn't always translate to getting that energy into a 
receiver usably.

Both Doug and Todd have commented on the merits of being able to locate 
active whips where they can avoid as much local noise as possible.  I 
totally agree. That's probably their single biggest operational benefit to 
most serious listeners in today's RF environment.

Yet there is also one more factor at work, considerably less obvious until 
you think about it, without which a small whip simply COULDN'T compare to a 
much longer wire.  It's a factor that also relates to the R-75 receiver 
thread, and applies to other general coverage receivers at LF.

While a few commercial antenna manufacturers might be happy for you to 
assume their whip designs are somehow magic (just as loop antenna makers 
like to have us call them "magnetic" loops as if that meant something), the 
real reason a relatively small whip CAN give good results in the real world 
is not because it has some trick for snagging more signal out of the air, 
and not because the amplifier has a lot of voltage gain--most aren't even 
unity gain--but because the impedance transformation to 50 ohms delivers 
more usable power to the radio's antenna input.

It's not that the active antenna snares as much signal voltage as a big 
longwire antenna is _capable_ of.  Not even close.  It's just that it 
doesn't waste virtually all of it through gross impedance mismatch!  Most of 
us have probably never thought  about the enormity of the loss that occurs 
the moment we terminate an antenna that's 1% of a wavelength or shorter to 
the 50 ohm input of a receiver, or upconverter, or even low-pass filter. 
Even connecting to a 500 ohm receiver input doesn't make a lot of 
difference...especially not at LF, where the matching transformer at the 
input is probably rolling off badly in frequency response already.

I used to buy the conventional wisdom that my Kenwood was just naturally 
deaf below 100 or maybe even 200 kHz.  When I actually measured its input Z, 
and the impedance of the real world antennas I had been using, I understood 
why.  It wasn't so much the radio, it was mainly the antenna.   As I hinted 
above, the 500 ohm input of the radio was scarcely 50 ohms at 60 kHz, and 
the 50 ohm input itself was also down several ohms.  The antenna I had been 
using for WWVB had a capacitive reactance of roughly 15000 ohms.  When I 
buffered my antenna, it's no wonder the signal came up something like 45 dB 
immediately.

The improvement was proportionally greater the lower I tuned.  Atmospheric 
noise was now plentiful all the way down to 30 kHz, even in the winter, and 
so were any stations that managed to be at or above the noise level.  Unlike 
some radios, the R-5000 has no automatic attenuator at LF...only between 500 
and 1600 kHz.  The path through the front end has almost no low frequency 
limitation except that of the first mixer input transformer, and it's pretty 
good to 30 kHz.  As for spurs, sure, there are plenty below 200 kHz with a 
dummy load or no antenna; and even a few annoying ones with some 
conventional antennas that I formerly used bareback.  The RF and IF stages 
were running at full gain with nothing else for them to hear but the spurs! 
As soon as I began buffering, though, and normal sferics became audible all 
the way down, you have to hunt hard to find a spur now.

As you enjoy your active whip at LF, Mr and Mrs LowFER, remember, it's not 
only helping you retrieve signal with less local noise intrusion, there's 
also the fact that it's helping you see it at your radio's antenna terminals 
without nearly the kind of losses that there would otherwise be.

John