[FARC] HF Gremlins

[Bob Moroney]

Kirk,

The choke balun that Joe describe could help solve some of your problem, 
but also keep in mind that even with a perfect transmission line setup, 
some of the RF radiating from the antenna will be induced into any 
conductive structure that happens to be nearby.   That includes your 
body and that of any other person (or pet) nearby, your downspouts, your 
house wiring, low-voltage control lines to your AC, audio system wiring, 
etc, etc.  If these items are electrically isolated from ground, they 
can even have a significant induced voltage while you're transmitting, 
to the point of someone touching, say, a metal downspout actually 
getting a jolt.

It's very likely that wiring like the control lines to your AC unit and 
your phone lines are not well grounded electrically.  Try listening on 
the phone while you're transmitting, and you're likely to hear a 
distorted version of your transmission.  You may not be able to fix 
these grounding problems, but you can buy some ferrite RF choke beads at 
Radio Shack or RF Connection or the next Hamfest and greatly minimize 
the problems caused by RF induced into this wiring.  You can clamp some 
of these beads around existing wiring, but they tend to be most 
effective when the wire is wound around them for a couple of turns.  The 
inductive characteristics of the ferrite at RF tend to attenuate the RF 
energy, but let the lower frequency signals pass.  If you put one of 
these ferrite beads at the AC end of the wiring, another at the 
thermostat end and one in the middle somewhere for good measure, your AC 
remote control problems may go away.  Same for any other such strange, 
RF-induced behavior in other circuits.   The coke bottle approach that 
Joe described is another way of "choking" stray RF off of the coax 
shield, where it can cause the mischief you described.  Slipping some 
ferrite toroids over the coax can also have the same effect.

You can look at an "antenna tuner" as a pacifier for the final output 
transistors in your Kenwood.  They want to see 50 ohms, period.  
Anything else, and they start to heat up, and that's not good.  If you 
have a truly resonant transmission line and antenna combo at the 
transmitted frequency then they'll see 50 ohms and dump all their power 
into the antenna circuit.  If the transmission line and antenna aren't 
resonant, the "tuner" adjusts the transmission line circuitry as best it 
can to give the finals 50 ohms and keep them happy, and passes whatever 
power is left over up the coax.  As long as you've got a tuner in the 
circuit, it will account for some degree of loss of radiated power.  
(Thus the "resonant antenna or nothing" school of thought.)

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 frequences 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
>