I have an 80 meter vertical/160 meter inverted L. It is very similar to
the antenna described in ON4UN?s Low-Band DXing book, (third edition.) The
primary differences are in the lengths I needed to use to achieve reasonable
The vertical component consists of 24 feet of tower (Hy-Gain Hy-Tower,
stripped of everything except the tower part) then 31 feet of 2 inch OD
aluminum tubing for a total vertical length of 55 feet.
The horizontal part of the antenna is about 40 feet of # 12 wire. An 80
meter home brew coaxial trap separates the vertical from the horizontal.
The radial system consists of 72 radials that average about 110 feet in
length, with the longest being 175 feet and the shortest about 45 feet. The
radials are laying on the surface.
The antenna is fed with 52 ohm coax. A series capacitor is switched in on
160. The 2:1 bandwidth on 80 is from 3.5 to 3.8 mhz. On 160 the bandwidth
is 1.8 to 1.85. These measurements are taken at the antenna feed point with
an MFJ Antenna Analyzer. The best SWR and Xs are at 3.650 and 1.825 mhz. on
80 and 160 respectively.
Note that the vertical part of my antenna is about 7 feet shorter than the
ON4UN book shows and the total length of the 160 component (V+L) is 50 feet
less. Extending the vertical length causes the low SWR and minimum Xs on 80
to go lower (as expected.) I extended the horizontal portion of the antenna
to 105 feet and kept shortening it until I could read a respectable SWR and
Xs on the antenna analyzer. I could not get a suitable reading until near
its present 40 foot length.
So while the antenna is working pretty well, I am curious as to why my
antenna seems to want to be so much shorter than the books call for. Also,
I assume I am losing some bandwidth, especially on 160 due to the shortness
of the antenna.
Any comments or suggestions would be appreciated.
From: Michael Baker [mailto:firstname.lastname@example.org]
Sent: Monday, November 22, 2010 7:12 PM
To: email@example.com; firstname.lastname@example.org
Subject: RE: [TowerTalk] Tune 160/80 Vertical
Don't think in terms of SWR. Think Rs with no Xs. Which means the
wire is too long. It is basically resonant at 1770. Not too unusual to see
impedances in the 16 ohm range with shortened antennas and that is what the
trap coil is doing.
Wind up an Un-Un that converts 50 to various impedances. I have one
I purchased that gives me a bunch of range from (If I remember correctly) 3
ohms to 36 ohms output range. If I remember correctly I got it from
CWSBytemark. It is a Jerry Sevik design.
Here is what I found that I have managed to copy and paste from the
POWER RATING: 1 KW CONTINUOUS, 2 KW PEAK.
UNBALANCED CO-AXIAL INPUT: 50 OHMS SO239
1 MATERIAL: UL RECOGNIZED 94V-0 FOR FLAMMABILITY
CORE: COATED FERRITE TOROID, K MATERIAL
CWS BYTEMARK OR OTHER APPROVED PART
WIRE: UL RECOGNIZED 200°C RATED MAGNET WIRE
MARK PART NUMBER AND REVISION
50 O : 12.5 O (4:1) CONNECT COAXIAL B(12.5 O) TO COAXIAL D (50 O)
50 O : 5.56 O (9:1) CONNECT COAXIAL C(5.56 O) TO COAXIAL E (50 O)
50 O : 4.08 O (12.25:1) CONNECT COAXIAL C(4.08 O) TO COAXIAL F (50 O)
50 O : 3.13 O (16:1) CONNECT COAXIAL C(3.13 O) TO COAXIAL D (50 O)
50 O : 36.76 O (1.36:1) CONNECT COAXIAL A(36.76 O) TO COAXIAL F (50 O)
50 O : 28.13 O (1.78:1) CONNECT COAXIAL A(28.13 O) TO COAXIAL D (50 O)
50 O : 22.22 O (2.25:1) CONNECT COAXIAL B(22.22 O) TO COAXIAL E (50 O)
50 O : 16.34 O (3.06:1) CONNECT COAXIAL B(16.34 O) TO COAXIAL F (50 O)
>From what you describe if you shorten the wire to resonance at the freq you
want (middle of where you intend to operate) and then use a matching method
(like this transformer matcher) you should be good to go.
If you want to have a good match AND 50 Ohms or there about, other methods
of tweaking the operating freq will be necessary. Tunable loading coil at
the bottom and switched in and out as needed for 160 operation.
Have fun... BTW, I use that matcher for mobile where low impedance is normal
and sometimes very low. 4 Ohms?
Michael Baker K7DD
[mailto:email@example.com] On Behalf Of firstname.lastname@example.org
Sent: Saturday, November 20, 2010 9:36 AM
Subject: [TowerTalk] Tune 160/80 Vertical
I'm helping a friend who has an 80 meter 1/4 wave vertical. At the top, he
has a 160 meter trap then a wire extending horizontally for 160 meters. It
is therefore an inverted L on 160.
The SWR on 80 is good and can be adjusted nicely by changing the length of
the vertical component. 160 meters, however is somewhat of a problem.
The minimum SWR he can achieve on 160 is about 1.7:1. Of course the object
here is to lower the SWR on 160. Here are some readings we took with my
MFJ -269 antenna analyzer in case this helps anyone.
1.770, Rs 16, Xs 0, SWR 3:1
1.800, Rs 27, Xs 15, SWR 2:1
1.825, Rs 45, Xs 26, SWR 1.7:1
1.85, Rs 79, Xs 31, SWR 1.9:1
1.875, Rs 128, Xs 2, SWR 2.5:1
Can anyone point us in the right direction to find a solution?
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