Re: Antenna for my Kenwood R-1000 receiver?

Larry wrote:

SNIP

Above 15 Mhz, it's somewhat of an antenna. Below 15 Mhz, the further down you go the worse it gets because it's just TOO SHORT! We put these awful lossy "tuners" between the bottom of the whip and the radio to match the impedance, at the cost of losing a lot of power. Being so
short at lower HF frequencies, its radiation pattern looks more like an inflated hot air balloon, radiating mostly straight up...not out towards the receivers over the horizon.

Actually, vertical antennas (even very short ones) radiate almost nothing directly overhead. Over seawater, vertical antennas radiate better at low angles toward the far horizon than just about any horizontal antenna.

These antennas at low frequencies have
almost no antenna CURRENT, which makes the required H field (the magnetic radiation part of the radio wave). They have lots of voltage, E-field,
but E-field cannot exist without H-fields perpendicular to them so they radiate poorly. You need BOTH.

If the short antenna is matched (i.e., you use a tuner) it will be nearly as efficient as its longer counterpart. A short vertical antenna has very HIGH current at its base. Any loss in efficiency is due almost exclusively to the tuner when operated over seawater. Over land, ground losses become a more significant factor in reducing efficiency.


Some of the "cure", about as much as you'll get, is to use a LONGER antenna with the tuner doing less of the work.

Ironically (as is often the case with nature: if bread and crackers are exposed to the same environment, the bread becomes hard and the crackers soft. Go figure.) at the higher frequencies, a longer vertical antenna may actually radiate less toward the horizon (low angles). So an antenna that is the "right" size for 4 MHz may not work as well on 15 MHz as a shorter one! An antenna "too long" for a frequency will develop lobes and nulls in the vertical plane which may be detrimental to your desired propagation path. Yet another need for thoughtful compromise.

One comes with sailboats.
We call it the "backstay", a nice sloping piece of wire we can insulate from ground on the bottom (series feeding it's called because the tuner is installed in SERIES between ground and the bottom of the backstay, because of the insulator. We insulate the top to keep the antenna current from continuing up the backstay into the mast and going DOWN the mast...which makes a wave that CANCELS the wave made by the backstay!

Well, if the mast is insulated at the base (some are) what you have is something like an inverted "V" (end-fed) and there will be some directionality, but cancellation is far too strong a term to describe it. Even if the mast base is grounded, the small amount of directionality would probably not be noticed in normal operation.

SNIP


On Geoffrey's former boat, an Endeavour 35 sloop, I used to load the port
shroud through the ungrounded chainplate that lead right into the port storage cabinet over the settee...and ran a ground wire down under the cushions to the ground strap back to the engine I installed. This arrangement SHUNT fed the mainmast, which wasn't grounded properly to anything at its step. Shunt feeding has been used since the beginning of radio. We hams have been shunt feeding out "masts" for decades:
http://www.qsl.net/w9rb/webdoc9.htm

Shunt-feeding the mast (and back- and forestays) doesn't require the base of the mast to be insulated. I've been shunt-feeding a grounded mast on a 34' Tartan for years and never felt the need for a better antenna. Since I use a manual tuner, it was impractical to feed the bottom of the backstay with a tuner in the cabin. Because the lower ends of the forestay and backstay are ungrounded, they act as a top hat, making the mast appear longer (electrically) than it is. Since the stays are not symmetrical, they doubtless provide some often needed horizontal radiation.

I must say that on some frequencies, particularly the 40 meter band, the feedpoint impedance of the system was wildly high. I ended up using some additional outboard reactances to tame it. Everywhere else, tuning was reasonable. An autotuner may have handled it OK. A key advantage is that you can install an SSB in minutes without worrying about insulating the backstay, etc.


Chuck



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