[NLRS] 2-meter TVI with a M2 Loop Antenna?

[Dr. Gerald N. Johnson]

On Wed, 2006-02-22 at 07:10 -0600, Carl Estey wrote:
> 
> For the past several years my sole horizontal 2-meter antenna has been a M2
> loop side mounted at 20' above ground on my 40' Universal tower - and for
> all of that time one neighbor about 120' away with a inside "rabbit-ears" TV
> antenna has called me frequently complaining that my transmitting is
> interfering with his sports events on local TV channels.  The Olympics are
> on now and his calls have become a nightly event. 

Could be worse if he's looking at cable channel 19. Seems popular for
sports and its 144 to 150 MHz. Takes the least leak in cables or TV to
get TVI.
> 
> I have experienced the same problem both with my ancient Kenwood TS-700A and
> the new Yaesu FT-897D (both in low power).  I keep telling the neighbor that
> the problem can be fixed but there never seems to be a time that is
> convenient for him.  His solution is that I never transmit - and frankly, I
> want to enjoy 2-meter SSB. I have a ferrite-bead core wound with RG-59 coax
> as a common-mode filter ready to try if he ever finds a time that is
> convenient.  

I don't think the common mode filter will help much, its most effective
at curing TVI from HF treating the antenna feedline as a long wire.
> 
> In discussion of the TVI problems with Vince K0VZJ, and he provided some
> email from Randy K9VHF and the following is from that email:
> "Also some antenna's seem worse than others. I have a stacked pair of M2 HO
> loops for 2 meters at 70+ feet. At 5 watts, those things throw more TVI than
> 50 watts thru the 17B2."
> 
> Has anyone else experienced TVI problems with the M2 HO loop on 2-meters and
> more importantly, has anyone found a fix?
> 
> There are still a pair of Cushcraft 17B2 antennas in me rear yard that I
> cannot put up due to the proximity of the tower to a large locust tree, but
> am hoping to get a single M2 2M12 up when it gets warmer this Spring.  While
> I might increase the field radiation off the front of the 2M12 as compared
> to the horizontal loop, I hope it will solve the loop TVI.  

The directivity of the yagi will put most of its RF up high over the
failing TV.
> 
> There seems to be something funny with the loop because I do interfere with
> the Sony clock radio in my bedroom even when it is off.  A common-mode
> filter on the AC line and various physical placements of the radio did not
> improve the RFI.  A second Sony clock radio in the same room does not have
> RFI.  The VSWR on the loop is near-flat! 

There could be a need for decoupling the coax braid outside from the
loop. There might be benefit to moving the loop to the top of the tower,
but the fundamental may be an overloading untuned preamp in the rabbit
ears as Jon Platt suggests because there is little directivity,
horizontal or vertical from the loop. Radiation from your coax can't
help and that may be responsible for your clock radio interference.
> 
> Any suggestions?
> 
> BTW, the 17B2 pair and the phasing harness/coupler remain available and for
> sale - maybe as it gets warmer, someone will get anxious for a really big
> 2-meter signal!  Give me a call.
> 
> 73
> Carl Estey WA0CQG
> wa0cqg at arrl.net
> (952) 250-3110 (Cellphone)
> 
The solutions may only work at the TV, but touching it can make you
responsible for all its future ills. A trap comes to mind, but if the
problem is a preamp in the rabbit ears, a trap at the tv input won't
help. 
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer