[FARC] HF Gremlins

Sun Feb 10 17:07:01 EST 2008

Yes Joe I've heard of the coiling of wire at the antenna feed point, which 
in my case would be at the top of my 30 ft. TV tower.  Supposedly it acts 
like a kind of balun but only as long as it is at the antenna feed point.  I 
have the extra coax from my 75 ft. feedline coiled up below the window where 
it enters the house but I've been told this will do nothing.  I could always 
try it though as it couldn't make things any worse.

As for tuners, they shouldn't be necessary and I don't understand the market 
for them.  For an antenna to radiate at the full power you expect it to, it 
has to be matched and where that perfect match occurs is the resonant point 
of the antenna.  In my case on 80 meters between 3.800 and 3.860 MHz.  The 
way I understand it now, in the resonant area of the antenna the reflected 
power is near zero, forward power is at or near the power you have set on 
the radio, and SWR near 1 to 1.  Anywhere else in the band is not in 
resonance, meaning one is not going to get full forward power out of their 
antenna; forward power minus reflected power.

So how bad can it get?  According to my ARRL "Practical Wire Antennas"  book 
an antenna on resonant frequency with an SWR of 1.5:1 will have negligible 
mismatch losses.   The author says it will take an SWR of 3.7:1 to double 
mismatch losses and he doubts that any wire antenna could be "so 
ill-fashioned or awkwardly positioned" that bad and that's why he recommends 
tuners.  In my case the SWR on the 40, 20, 17, 15, 12 meter bands pinned the 
SWR needle, way over 3:1 so mine must be "ill-fashioned.  Sure, a tuner will 
get the SWR down to a tolerable level, but I'm probably not radiating 
anything.

The antenna tuner is a sly beast.  The radio thinks everything is matched, 
and the tuner's meters show that forward power is up near where it is 
supposed to be.  But it just can't be.

It's interesting though.

73
KB3ONM
Kirk

>From: "Joseph M. Durnal" <joseph.durnal at gmail.com>
>Reply-To: "Frederick, Maryland ARC" <farc at mailman.qth.net>
>To: "Frederick, Maryland ARC" <farc at mailman.qth.net>
>Subject: Re: [FARC] HF Gremlins
>Date: Sun, 10 Feb 2008 11:58:26 -0500
>
>Hi Kirk,
>
>I've mitigated a similar problem on dipole and vertical antennas by
>building a RF choke out of an empty 2 liter bottle with 10 turns of
>RG8X coax.  I just used duct tape to hold it all together.  It might
>not look like much, but it sure helped me out.  There are two thoughts
>on where to put this contraption, the first being at the antenna feed
>point, which would keep the RF off of the feedline completely, the
>other is at the point where the feedline enters the the house, the
>outside feedline can still radiate but the inside feedline should be
>OK.
>
>This should also help with SWR readings and tuners.  Sometimes the
>radiating feedline can cause anomalies in SWR readings, making it
>difficult for an autotuner to get a match.
>
>73 de Joseph Durnal NE3R
>
>On Feb 10, 2008 11:48 AM, Kirk Talbott <KirkTal7237 at msn.com> 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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