[Boatanchors] antenna question

[Jim Wiley]

The 75-ohm distribution system may work just fine.  The caveat has to do 
with the "splitters".  The cable itself will carry the signals just 
fine, and with minimal loss.  The small additional loss caused by the 
(approximately) 1.4 to 1 SWR system when feeding a 50-ohm termination 
(in this case one of his receivers) from a 75-ohm source will not be 
noticeable at HF for receive signals.  In all likelihood, the HF 
receivers your friend has are unlikely to be precisely 50-ohms resistive 
at their input terminals anyhow.

The splitters at each distribution point may be a problem, maybe not. 
Depending on their internal circuitry, they may work well at MF and HF, 
or they may not.  There is no way to tell without testing or at least 
trying them to see.  If they work, with relatively low loss, then all is 
well.  If not, then you may have to look for some better quality units.  
Some of the units designed for broad-band cable systems work well at HF 
and MF, others do not.  Most types designed for just TV antenna 
distribution will probably not work well.  Again, try them and see.

If the splitters in place  do not work well (characterized by very low 
signals), then you can try several alternatives.  (1) try direct 
connection (a straight direct-connected coaxial tee fitting, with no 
splitter). If the various receivers don't try to talk to each other (HF 
oscillator interference) a simple hookup like that may be all you need. 
   (2) Try resistive splitters. These will have 10 to 15 dB loss, but 
that may be acceptable, considering that you are working from an active 
receiving antenna that has some gain that can overcome losses down the 
line, and resistive splitters are inherently very broad-band. (3) Roll 
your own couplers from ferrite toroidal transformers - a reasonably easy 
process that is not expensive, and works well.  Suitable designs can be 
found on line or in many of the later handbooks. You will most likely 
want a hybrid-type coupler that has about 6 dB insertion loss.

If I was dong this, I think I would prefer the resistive splitters over 
building hybrid transformers.

The fact that he cannot receive signals well inside the building is 
almost certainly due to the fact that the metal siding, particularly if 
well bonded, is acting like a shield and preventing HF and MF signals 
from getting inside the structure. MF and HF signals have too large a 
wavelength, and the holes (doors and windows) in the structure are not 
large enough to admit the signals.  This is the same effect that causes 
AM broadcast signals to fade when driving under overpasses or inside 
metal bridges (or tunnels) but FM signals continue just fine.  The 
roughly 3-meter wavelength of the FM signals "fit" though the holes in 
the structure, the several hundred meter wavelength of the AM BC signals 
does not.

He will definitely need some sort of outside antenna, either amplified 
or perhaps just a plain wire.   If he wants to get fancy, he can install 
a distribution amplifier from the main antenna to his former CATV 
distribution system to boost signals as needed.  Needless to say, he 
will not be able to transmit through such a hookup, but from what you 
say, he seems to interested in receive only.

In any case, I would leave the installed 75-ohm cable system in place.  
One never knows when such a thing may come in handy.


- Jim, KL7CC



On 1/4/2013 4:21 AM, Gerry Steffens wrote:
> An acquaintance who is also a boatanchor collector/SWL/BCB DXer (and messes
> with longwave a little bit) asked me the following antenna question.  It has
> been a while since I have sharpened up on antennas so I said I would ask the
> wizards.
>
> Wizards:
>
> He recently moved from a plain old house to a quite new residence with metal
> siding.  He reports that reception has gone in the tank compared to his old
> place.  He has various radios scattered throughout the house and had
> previously connected only a chunk of wire to most and received a good
> quantity of signals.  He also uses several McKay/Dymek active loop antennas
> (one per radio) on some of the BCB radios.  None of these work well in the
> new place.  He uses a McKay/Dymek (Stoner) outdoor active antenna (DA100D)
> at his bench.  The latter works very well.  The DA100D has an amplifier at
> the outdoor antenna and another inside at the bench.  Output of the base
> unit is RG58 cable to the receiver in use.
>
> The local TV-DSL/internet provider has gone digital and recently came
> through and replaced their old system box with a new one at the entry point
> and ran CAT 5 cable from their new box to another "BOX" at each TV set and
> computer, abandoning his 75 ohm analog distribution system which runs all
> over the house.  The question: Is there a good way to connect the active
> antenna output which uses RG58, 50 ohm output  to utilize the 75 ohm system
> to get an antenna to his several radios and expect decent receiving
> performance?
>
> He came up with this as he was preparing to remove the old abandoned system.
> He has decided that if he can't use the 75 ohm system, "Its gotta go!"
> Frankly, he has more ambition than I as I would just let the old system
> collect spider webs.
>
> Gerry
>
>
>
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