[FARC] HF Gremlins

Sun Feb 10 15:12:29 EST 2008

Kirk,
One other thing I wanted to mention in my earlier screed, but forgot, is 
that "electrical ground" and "RF ground" are mostly two very different 
things.  In the rare circumstances where a perfect electrical ground is 
also a perfect RF ground, it's usually either because of some fortunate 
coincidence or because the whole grounding system is very carefully 
engineered. 

Just from an electrical safety point of view, you should confirm that 
all the outlets in your house are wired correctly and that there is a 
good, solid connection between a stout copper wire that goes from your 
electrical panel to a stouter copper ground rod driven into the ground 
near where Allegheny's feeder line comes into your electrical meter.  If 
you don't already have one, you can buy an inexpensive AC outlet tester 
at any hardware store that you plug into each outlet in the house to see 
whether it's wired properly.  If you get any faulty readings from any 
outlets, you'll need to fix them or risk electrical shocks and/or other 
electrical problems. 

While you made a valid point about long grounding wires sometimes 
causing more problems than they fix, I think it's still a good idea at 
least to make sure that all the components in your shack are 
interconnected with a single external "ground bus", in addition to any 
power cord ground connections.  You can make one using 1/2" or wider 
flat tinned wire braid that interconnects the ground screw on every item 
in the shack: transceiver(s), power supply(ies), tuner, coax switch, 
etc, then connects to a good electrical ground.  You could use the screw 
on an outlet plate if nothing else is available.  These interconnections 
are usually fairly short, and the use of flat braid helps to minimize 
any induced RF.

Of course, the other thing is to unplug your gear and disconnect it from 
antennas during thunderstorms (or any time it's not being used, for that 
matter).  It doesn't hurt to have a coaxial switch or other connection 
that will ground the antenna when it's disconnected from the rig, just 
to drain off any stray energy induced by lightning in the area, or even 
by high winds, which can build up high voltage static charges on the 
antenna wire.

Good luck in taming your "gremlins".

73, Bob K9CMR
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Kirk Talbott wrote:
> I've had some interesting revelations about HF this week and a gremlin so I thought I'd offer them up to the group for some advice.  I've already had some good answers but I'm looking for a consensus.
>
> First of all the station set up.  A Kenwood TS-2000 transceiver, an 80 meter inverted Vee dipole high point about 30 ft..  75 ft. of RG-8X coax feedline to a window and a 25 ft. RG-8X coax run in the shack to the radio.  The Kenwood has its own internal auto tuner and I also use an LDG AT-100 auto tuner.  None of the radios or tuners are connected to a ground, more on this later.
>
> Am I getting out?  In a great discussion with K3ARN and W3ICF about forward power, reflected power, SWR, antenna resonance, antenna/feedline impedance mismatches, and antenna analyzing equipment, they offered some excellent ideas about how to determine where your antenna is most resonant without using antenna analyzing equipment.  To most of you pros out there these ideas will seem obvious but to a rookie they were a great help and their best idea was to record SWR meter readings at various frequencies on various bands and make a chart or plot of what was going on. 
>
> What I found was that my SWR was 1.5 to 1 or lower on the 75-80 meter band frequencies of 3.800 to 3.860 MHz. and this without an antenna tuner of any kind and right in the middle of the General class phone part of the band, perfect.  Above 3.860 MHz. or below 3.800 Mhz., the SWR instantly climbed to 2 to 1, 3 to 1 and beyond.  Ok, it's an 80 meter dipole and it works great on 80 meters, duh.  Now that this works I thought, lets see if I'm to be rewarded with any other operable band of frequencies by the antenna gods.  Not to be.  The plot also showed the other HF bands, 40, 20, 17, 15, 12, and 10 meters had totally unacceptable SWR, with readings that were full-scale off the SWR meter in the radio.  Antenna/Feedline impedance mismatch. THE REASON YOU CAN'T HEAR ME IS BECAUSE OF AN ANTENNA/FEEDLINE IMPEDANCE MISMATCH!  Somebody should make a T-shirt with this on it at the Ham Fest this summer.  
>
> Now enter the antenna tuner, a device whose real name should be "LIAR."  Sure, the antenna tuner will tune all the bands to a 1 to 1 SWR with the exception of 40 meters which tuned to a 1.5 to 1 SWR.  Everything is beautiful, right?  Now I can throw 100 watts at my multi-band antenna and work all the bands and all my 100 watts will hit the antenna and be radiated out into the ether.  Nope, not even close.  I can work 75-80 meters 3.800-3.860 MHz. when there is not a good-ole boy with a linear amp on every frequency, and I can work my neighbors on Ridge Road and in Frederick on 10 meters.  But generally I can't work anybody with a good strong 599 signal on 40 thru 10 meters at long distance, and some not even at short distance.
>
> The gremlin.  Ok, I'll settle for 80 meters on HF as long as I can to talk to somebody and they don't have to struggle to hear me and I'm as happy as a fly on.....well you get the idea.  So I ramped up the power on the Kenwood to first 75 and then 100 watts and called CQ and what happened?  The air handler (fan) for my attic air conditioner unit came on.  First on 3.850 Mhz., then on 3.830, and on 3.820 and just about anywhere in the band where I wanted to work.  Did it do it at 5 watts during my SWR testing?  Nope.  Was the air conditioning system actually off?  Well this isn't so clear cut.  The air conditioning thermostat has two settings for the air handler fan, ON and AUTO, there is no OFF setting.  My thermostat was set on the AUTO setting and my 80 meters works as a great remote control.  I did make a contact though while huddled in front of my radio freezing with the air conditioner running.
>
> After mentioning this fact to several Hams, some of whom looked at me as if I were crazy, thought a moment and then came up with RF ON THE FEEDLINE.  Where does this come from?  ANTENNA/FEEDLINE IMPEDANCE MISMATCH was one answer.  Another answer was inductance tripping an on/off relay in the air handler unit.  Another was in the form of a question. Are you grounded?  No, I don't have any ground wires going from the radio to a ground.  Why?  I read in a book that one, most modern houses have properly grounded electrical outlets such that running ground straps to radios wasn't necessary anymore and two,  running an excessively long ground wire could actually serve as a detriment because it would act like an antenna an bring RF into the shack.  What is an excessively long ground wire?  In my case it would be 25 ft. or more since my XYL wouldn't appreciate a bare copper wire running through the middle of my shack's (bedroom/den) floor over to the window.  I would have to run my ground wire around and behind furniture along a baseboard.  But the books say ground wires should be as short as possible.  Give me a number.  One book said 5 feet or less.
>
> So, I would appreciate any and all ideas that you may have.  
>
> 73
> KB3ONM
> Kirk
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