[Boatanchors] Antenna relay questions

[Donald Chester]

Rob Atkinson <ranchorobbo at gmail.com> wrote:

> I always advise getting rid of unbalanced tuners,
> baluns and so forth, and run ladder line from the center feed point to
> some sort of outer wall entrance.  

The all-too-common practice of running an unbalanced tuner output through a balun to a balanced tuned feedline is very poor engineering.  By definition,  tuned feeders operate with a significant standing wave.  The transmitter/tuner end of the feed line may be highly reactive, and the resistive component is apt to vary widely over the frequency range of a single band and from band to band.  Baluns, OTOH, are designed to work to a specified resistive load, usually 50 or 75 ohms, and only rarely will the balun see this load.  Force-feeding a reactive load at a considerable mis-match from the nominal load impedance the balun was designed for, is asking for trouble.  This may result in over-heating, damaging the balun (I have heard of cases of the balun actually catching fire), and reduction of efficiency (any RF energy wasted heating the balun is energy not delivered to the antenna).

The balun, if used, should be inserted between the transmitter and the tuner, not between the tuner and the antenna.  A classic balanced link-coupled tuner is inherently its own balun and probably doesn't need one.  If all you have is one of those unbalanced input to unbalanced output L- or T- network tuners, the whole thing may be used satisfactorily, "floating" it from ground by mounting it on stand-offs, a sheet of clear plastic, or some other form of RF insulation. Then, insert the balun in the line between the transmitter and the tuner.  The entire tuner enclosure (if it has one) will be hot with RF,  so a well-insulated tuning knob is recommended, and keep the tuner well clear of other equipment and  human bodies.

> Somewhere outside not too far from the wall but not too close,
> splice in a pair of banana plugs and jacks into the line.  This is
> where you disconnect the line and pull it away from the wall when away
> from home or during bad wx. 

That will work, but in my present shack, I use wall feed-throughs designed for the purpose (hamfest finds), employing glass bowl insulators and threaded copper rods.  This required boring holes through the wall.  At a previous location I cut a small rectangular hole in the wall, and fed the OWL through two sheets of 1/4" thick plexiglass mounted both inside and outside.  If you don't want to cut or drill holes in the wall, it may be run through a window, by replacing one of the glass panes  with a sheet of Plexiglass or Lexan. Pass the feedline conductors through holes drilled in the plastic sheet.  Another method is to use a piece of wood or vinyl, about 4" wide and long enough to fit across the bottom of the window.  Feed the line conductors through this strip (using insulators in the case of wood), and let the window back down on to this strip.

For lightning protection I use one of those large Signal Corps two-conductor knife switches often seen at hamfests.  It's mounted on the inside wall just under where the line enters. When not in use, the feed  line is grounded to a large  copper conductor, or better still a wide piece of copper  strap, which leads to an earth ground immediately outside the house beneath where the feedline enters. I have used this system for more than 50 years and seen some pretty severe lightning storms, and never had any lightning damage. It might be easier to mount the knife switch could be mounted outside, but I would recommend some kind of protective shelter if the switch has any parts of phenolic or material other than porcelain.

Lightning surges  have short rise and decay time, making them more like RF than DC or regular 60~ a.c.  Besides the standard 8' ground rod, I run several buried radials made of #10 copper wire, bonded to the ground rod, each about 20' long.

> Don't use the crummy brown
> plastic "window line" in any case.

I agree.  It is inherently less efficient than real open-wire line, but that in itself probably isn't enough to be noticeable with the signal at the other end.  The main drawback to the "crappy brown stuff" is that it is affected by rain, snow and ice, often enough to de-tune the antenna while it's raining, and ice and snow may de-tune the system enough to render the antenna unusable, plus the accumulation water and ice further reduces the efficiency even if the antenna is still functional.

One antenna change-over relay is enough.  It should be inserted in the line feeding the tuner, so that the receiver, as well as the transmitter, benefits from the action of the tuner.

Don k4kyv