[TowerTalk] 40M rotary dipole

[Jim Brown]

On 9/20/2017 3:29 PM, Jim Thomson wrote:
> Date: Wed, 20 Sep 2017 16:42:42 -0500
> From: Stan Stockton <wa5rtg at gmail.com>
> To: Dave Sublette <k4to at arrl.net>
> Cc: Dan Maguire via TowerTalk <towertalk at contesting.com>
>
> <A question regarding chokes for K9YC.
>
> <Let's assume a coax going to an antenna is buried to the tower, the shield is grounded with a short length at the base, the coax is inside the tower and the shield is electrically bonded to the top of the tower and anything else that would be considered good <practice for a good RF and lightning ground.
>
> <How is common mode noise making its way to the feed point and back to the receiver if you have something less than multiple cores at the feed point of the antenna?
>
> <Stan, K5GO
>
> ###  CM  goes up the tower faces to the top of the tower, then to the ant...then back down the center conductor.

Right -- sort of.  It's not a DC circuit, it's RF, so the tower and coax 
shield are parallel conductors that act as part of the antenna.  If 
there's an external field, the tower/coax will be excited as an RX 
antenna, current couples to the antenna if no choke.  All of this, of 
course, is very sensitive to frequency/wavelength.

> ##  Easiest way to see if the CM  choke is  doing the job, or to compare  no choke, lousy choke, good choke,
> is to use a clamp on RF ammeter, like the deluxe version that MFJ sells.  MFJ-854.   Measure it at the base of the tower,
  Sorry, that's not valid, because like any conductor carrying RF 
current, the current varies along the length of the conductor depending 
on its boundary conditions ( the mathematical term for its 
terminations).  These boundary conditions are sort of obvious -- current 
is forced to near zero at the end of an unconnected wire and at a choke 
with very high Z. But just because the current is near zero at the top 
doesn't mean it's near zero anywhere else along the length, because it's 
an ANTENNA!   SO -- a current measurement at the tower base does NOT 
tell us about current at the top next to the choke.

How much noise drops depends a lot on how much noise you have. :) If you 
don't have much, a choke won't do much. :)

Another important function of a choke is to minimize coupling between 
antennas in a multi-transmitter setup, where the coupling mechanism is 
radiation from one coax and reception on another, or reception by 
another RX antenna.  Multiple chokes on a a vertical run of coax to a 
high dipole can act as "egg insulators" to prevent that coax from 
becoming a parasitic element for a nearby vertical antenna.

The fundamental principle here is that depending on lots of variables, 
it's entirely possible for various antennas and feedlines in any antenna 
farm to "see" each other and vary each other's patterns. These effects 
can be a little or a bit more. They're rarely large.

Example -- I have a 120 ft tower holding a SteppIR and a long 2M Yagi, 
and it's about 200 ft from a 160M Tee vertical that has a 100 ft 
vertical section. When I first installed the tower, both NI6T and N6BV, 
separately, told me I should look for interaction between the tower and 
the Tee. Putting the dimensions in NEC, I found that the tower acted as 
a passive reflector for the Tee, giving me 2-3 dB of gain to VK/ZL. I 
also rigged a pair of sloping verticals to the east and west of the 
tower, insulated from the tower and fed from their bases against 
elevated radials. The tower acts as a passive reflector, and gives me 
about 2dB in th0 direction of whichever of the two sloping wires I 
feed.  AND -- if I ground the Tee, it acts as a passive reflector for 
the two sloping wires, rotating their patterns about 40 degrees 
northward, the east wire to EU, the west wire to JA. On the air results 
clearly confirm the accuracy of the careful models -- as I switch 
between the three antennas, i can clearly hear the directivity.

ANY gain on 160 is hard to come by, so when it comes almost for free 
simply by recognizing it and switching it in and out, it's hard to turn 
down. :)

73, Jim K9YC