If one's tower does not have radials, is set in concrete, with minimal
direct contact with earth (say less than a linear foot, with nothing more
than the usual lightning treatment attached to the base, then the base
connection is likely rather resistive RF, and strictly by itself the tower
is a fairly impeded conductor at 160. The conductors you need to block
current on are those going up the tower: coax, control leads, and possibly
the lightning ground leads. You can get that done with various
applications of #31 ferrite. Toroids, monster clamp-ons, and beads,
without getting into specifics.
Since house radio garbage stuff frequently comes out coax shields to and
then up towers, it is not too hard to get a permanent infusion of junk into
just about any RX antenna on a small lot and wind up blaming it on the
antenna.
If you put a block on **ALL** the coax and control conductors just before
they go up the tower, you may solve a lot of problems. For 80 and 160,
seven or eight turns of RG400 through a five stack of #31 ferrite FT240
form factor toroids will put a hefty block on stuff off the tower. Simply
running eight turns of rotor cable through a five stack will block all the
conductors, whether used or not. A good article on how this works is at
http://audiosystemsgroup.com/RFI-Ham.pdf Do a search on #31 to go directly
to the relevant material.
We are developing some special uses of an FCP to substitute for radials
where towers on a small lot either are the radiator or support the
radiator, as in an inverted L. This allows one to pull all connections to
the base of the tower other than lightning protection (through expendable
#31 ferrite). These FCP methods will require current blocking treatment
of ALL conductors on the tower. This will at one time substantially reduce
noise induction, and reduce loss caused by these conductors becoming
default lossy radials and significantly decouple all of them from the RX
antennas. An FCP, its isolation transformer, and L supported by the tower,
is detuned from affecting the RX antenna by breaking the contact between
the isolation transformer's radiator connection post and the L.
Rather than looking at it all as an unfortunate discovered disadvantage,
look at it as there IS a way. It's just to do some stuff on a small lot is
going to require a bunch of #31 ferrite. Cheaper than moving :>)
73, Guy.
On Wed, Jun 20, 2012 at 3:45 PM, Herb Schoenbohm <herbs@vitelcom.net> wrote:
> Tom,
>
> This is not encouraging news for those of us with towers already ground
> and are either shunt fed or cage fed. Even though they are not
> resonate, although top loaded and approaching resonance, it seems that
> even with some elaborate decoupling arrangements, and I hope I am wrong,
> not to much can be gained by even trying to get them "detuned". This is
> crucial for me to know, and I would assume many others that have
> Beverage terminations or feed points 100 feet away,
>
> Thanks for the warning as this summer I was going to install a HV relay
> on the shunt cage to ground....but as you point out it may not help as
> the grounded shunt or cage fed tower is already a noise source for close
> proximity RX antennas.
>
>
>
>
> On 6/20/2012 2:09 PM, Tom W8JI wrote:
> > There are two potential problems with this. As general rules:
> >
> > 1.) Grounding any antenna which is dependent on a ground system to be
> > resonant will maximize reradiation.
> >
> > 2.) Resonant elevated radials, even without an antenna connected, are
> > resonant and re-radiate.
> >
> > 3.) Things that are not resonant can still re-radiate.
> >
> > To minimize coupling from a resonant radial, the radial has to be
> > disconnected from ground and from other radials.
> >
> > Different systems can be different, and in some cases re-radiation can
> > actually cause a null that reduces noise, but the general rule is
> > self-resonant antennas with a ground (Marconi), or nearly self-resonant
> > antennas when grounded, should be floated. Other antennas can be
> terminated
> > in an inductor, capacitor, or opened, depending on the system.
> >
> > My 220 ft insulated tower, in the center of a four square, is "opened" by
> > shorting a specific length of coax to the "L" matching network. It has
> much
> > more radiation when open or grounded. radiation is minimal when detuned
> by a
> > proper impedance.
> >
> > 73 Tom
> >
> >
> >
> >
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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