Personally I typically evaluate any HF antenna that I use based on a 3:1 SWR
bandwidth. That better is defined as being the frequency that produces a
3:1 SWR below the 1:1 SWR frequency and the frequency that produces a 3:1
SWR above the 1:1 SWR frequency. Using an Antenna Bridge such as the
MFJ-250 will do an excellent job finding these points.
The reason I choose a 3:1 SWR value is the fact that Tentec PA circuits do
not have any issues with a SWR of this value or lower. I can't say the same
for other brands. And I find that any reasonable feedline, discounting open
wire lines, will have negligible loss with a 3:1 SWR or less.
All please note; Jim K9YC has a very good explanation of radiation
resistance, wire resistance, and ground resistance and such in his post.
73
Bob, K4TAX
----- Original Message -----
From: "Jim Brown" <k9yc@audiosystemsgroup.com>
To: <tentec@contesting.com>
Sent: Wednesday, November 06, 2013 6:00 PM
Subject: Re: [TenTec] 160 M antenna
On 11/6/2013 3:34 PM, resluder@yahoo.com wrote:
What kind of bandwidth do you get with your inverted L, 50 kHz? Would I
still be able to use a tuner in the shack if I want to bend it for more
coverage on 160?
Define bandwidth. Do you mean SWR bandwidth? Do you mean you can get it
to load?
If you can get the total wire length > 100 ft and connect it to a decent
radial system, a decent tuner should be able load it across all of 160M,
but you'll need to retune it as you QSY. The Ten Tec 229 and 238 series
tuners work very well on 160M with reasonably good antennas.
I have a Tee vertical for 160M that's 86 ft vertical with roughly 100 ft
on top. That makes it longer than a quarter wave, and I've adjusted the
length of the top section so that the feedpoint Z is 50 ohms resistive
plus some inductance (because it's electrically long). I then added
series C to tune out the L, giving me a nice match to 50 ohms. I can work
at least the bottom 75 kHz of 160M without much tuning.
A well known way of broadbanding almost any antenna is to make the
conductor fatter. An easy way to do that is to use two spaced conductors,
tied together at both ends, and that's what I did. The vertical is two
#10 THHN wires spaced about 6 inches. I did that, and the SWR bandwidth
improved by at least 50%.
As to loading a 45 ft wire on 160M, which Jim asked about. I'd guess that
the Ten tuners will LOAD it, but the radiation resistance of such a short
wire is pretty low, so it won't be very efficient without a great radial
system. Remember that a vertical is a simple series circuit --
radiation resistance (Rr), wire resistance (Rw), and ground resistance
(Rg). The radiation resistance is the "good" resistance -- it accounts
for radiated power, but the same current flows in all three resistors, so
the power divides between them. For the antenna to WORK, we want Rr to be
much larger than (Rw + Rg), but it takes a lot of wire to get Rg below 10
ohms on 160M, and Rr of a 45 ft vertical won't be much more than perhaps 6
ohms, yielding an efficiency of about 30%. That means your 100W radio will
put 70W into the ground and 30 watts into space. .
73, Jim K9YC
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