From: dale@sequent.com (Dale Mosby)
Newsgroups: rec.radio.amateur.misc
Subject: need help with TVI problem
Date: 10 Nov 92 07:28:16 GMT
Article-I.D.: sequent.1992Nov10.072816.18595
Organization: Sequent Computer Systems Inc.

I am quite confused as to why I am unable to solve some TVI problems.

The short problem description is that when I transmit on 10 meters I
cause interference on channel 2 and a low pass filter does not correct
the problem.

Sorry for the length of the posting.  I have been working to solve the
problem for a long time with no success and I'd like to stop fussing
with the station set-up and just get back on the air.

This problem first came to my attention when a neighbor mentioned that
I was causing some TV interference.  I found that I do indeed cause
moderate to severe interference to channel 2 (moderate but unacceptable
with 100 watts and severe with 500 watts).  Next channel is 6 and no
problems there.  This is with an external antenna ("rabbit ears" or
outside antenna) no problems for those people on cable.

For test equipment I have used a Motorolla Communications System
Analyzer Model R2600-B and a Hewlett Packard 8590A Spectrum Analyzer --
thanks much to a good friend.

My rig is a Kenwood TS-430S and the problem shows up on 28 meters.
I've been doing my tests on 28.500 so the second harmonic is visible
between the video and audio of channel 2.  Transmitting on 28.5 the 2nd
harmonic is down 32 db which is not quite within specification for this
radio.

Transmitting on 14.250 the 2nd harmonic is down 38 db and the 4th
harmonic is down 42 db.  This is barely noticeable, but acceptable on
channel 2 if running 100 watts.

These measurements were made by transmitting into a dummy load which
providess a tap for an RF probe (Heathkit "cantenna").

When transmitting into the dummy load I do not interfere with the TV,
and even when transmitting into an antenna I do not cause TVI which
indicates to me that the problem is with a transmitted signal in the
channel 2 region, and not with signals on the power line.  A high pass
filter on the TV also does not help, leading me to believe that the
problem is not with front end overload.

I have observed the problem on two color televisions at my house using
an attic antenna, as well as a small black and white portable powered
by a battery.

My station originally had a reasonable ground, but after discovering
the problem I remade all cables connecting tuners, meters and such
using 9913 cable.  I also installed a ground bar on the floor below
equipment.  This is a 4 foot by 1/4 by 2 inch copper bar.  Each end of
the copper bar is connected to an 8 food ground rod just outside the
house with one of these further connected by an undergound wire to
another ground rod about 4 feet out in the yard.  All station equipment
is connected by heavy braid to the copper bar.

I also installed two (count 'em 2) Bencher YA-1 low pass filters, one
just after the radio and the other just after the amplifier.  I also
have on and a borrowed low pass filter which is presently connected,
making a total of 3 (yes three!) low pass filters.  These low pass
filters are bolted to the copper bar.

I have read the ARRL RFI book which is quite good and have tried things
like removing all my tuners/meters/etc. and running the radio directly
into an antenna (R7 vertical or A3S beam) as well as through a low pass
filter into the antennas.  I still suffer the TVI problems.

Setting up the Motorolla analyzer as a tracking generator I was able to
examine the filters and determined that they all meet specifications.
Here is partial data for one of the bencher filters:  37.0MHz  -3dbm,
41.5MHz -10dbm, 44.5MHz -30dbm, 47.7Mhz -50dbm 49.2MHz -60dbm, 50.4MHz
-70dbm, 51.3MHz -80dbm

The filters seem to work, but when we look at the signal as seen either
by tapping off the dummy load, or as picked up by my TV antenna we see
only 3db attenuation from the filter. I do not understand what is
happening here.  Is the signal on my coax shield?  I even tried
slipping 8 type 77 and 8 type 43 ferrite beads over the coax.  I also
tired wrapping a length of RG8X around a 2 inch type 43 ferrite core.

Hoping that I could solve the problem by telling my wife that I really
needed a new Kenwood TS950S (with DSP option) for Christmas I decided
to try another radio.  I borrowed a friends Kenwood TS440S and found
that it also caused interference on channel 2.  I did not have the test
equipment available for actual measurements of harmonics.

I then visited a friend with a brand new ICOM 735.  He as a multi-band
dipole and tuner and we found that when he transmitted on 28.5 he gets
into his TV using an indoor antenna.  No problems reported for his TV
on cable.  My radio at his house had the same affect.  His antenna is
quite close to his house, so I borrowed his radio and tried it at my
house.  The problem occurs running his radio on my set-up (all 3
filters) as well as running his radio straight into a filter and then
into an antenna, bypassing all other station equipment.

Well I know this is a long post, and if I've overlooked something
really obvious I'm quite willing to suffer public embarasment just to
get the problem solved and get back on the air.  Thanks.

-- 
  dale@sequent.com         OR      uunet!sequent!dale
  Dale Mosby   503-578-9842  N7PEX  // Sequent Computer Systems, Inc.
  15450 SW Koll Parkway             // Beaverton, Or.  97006-6063