[Boatanchors] Short Wave Broadcasts

[Richard Knoppow]

----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: "Boat Anchors List" <boatanchors at mailman.qth.net>
Sent: Friday, April 18, 2014 11:25 AM
Subject: Re: [Boatanchors] Short Wave Broadcasts


>I second the verdict on one sideband plus carrier.  Every 
>one I have
> heard sounded poor and distorted.
>
> Re the curtain antennas--they are wildly expensive to 
> construct and
> maintain.  Every privately owned swbc I have ever heard of 
> has run
> rhombics on telephone poles if the tx site is in the US. 
> Elsewhere,
> costs may be different.  Here, dipole curtains seem to be 
> affordable
> only by VOA.
>
> 73
>
> rob
> k5uj


      Some time around the 1960s a fellow named Leonard Kahn 
began making a system for transmitting SSB through an AM 
transmitter.  He wrote a number of technical papers in peer 
reviewed journals and has a pile of patents.  He based his 
system on the theory that all modulation is a combination of 
amplitude and vector modulation (I have that term wrong but 
can't pull the right one out of my head).  Essentially the 
system converts whatever is put into it into a frequency 
component and an amplitude component. Both can be put 
through an AM transmitter. When the outputs are combined 
they reform whatever the original signal was. I believe the 
VOA used some of these things on their transmitters and may 
well have used them for single sideband with carrier. At 
least two local broadcast stations used the Kahn system for 
SSB with carrier; one was KBIG, which as that time was a 
10KW daytime only AM on Catalina island (at 740Khz).  The 
other was a station somewhere like San Bernardino that was 
causing monkey chatter on an LA station. They also used SSB 
with carrier to remove the modulation products that were 
causing problems.  One or both of these stations also used 
off-set carrier so that the channel center was on the 
assigned carrier frequency.  KBIG was always funny sounding 
as was its FM counterpart KBIQ.  The owner was also the CE 
and who knows what odd-ball stuff was in the air chain. 
KBIG, now KBRT had an antenna with a large reflector built 
on the side of a cliff. They covered an enormous area with a 
weak but steady signal.  They lost the lease on the property 
and have moved to Costa Mesa in Orange County. The 
difference is that they now interfere with KCBS at night.
     One can make a creditable antenna using the "lazy-H" 
arrangement.  This is a very simple curtain antenna, 
essentially two half-wave dipoles stacked with a phase 
inversion between top and bottom. It is bi-directional but a 
second antenna of similar type can be placed just behind it 
as a reflector. This gives a reversable pattern with the aid 
of a relay.  Lazy-H antennas have reasonably high gain and 
are reasonably economical to build.
     Rhombic antennas can work very well but have the 
disadvantage of requiring a lot of realestate.  The pattern 
is affected by other antennas or conductors like power lines 
that are at some distance.  The need for a dissipating 
termination can be eliminated by using two rhombics on the 
same poles but spaced a little with one being a reflector 
for the other.  The price is a reduction of bandwidth. 
However, the bandwith of a rhombic is mostly a matter of 
constant impedance, the directional pattern varies 
substantially with change in frequency.  A good book on 
traditional type antennas, including rhombics is "Antennas" 
by Edmund La Port. La Port was an engineer for RCA for many 
years. The book has photos of things like huge insulators 
for 100KW antennas.

--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com