A couple of notes about using a 80 meter dipole (center fed 130 feet long) on
multiple bands fed with ladder line (open wire).
Using a good open wire tuner - results in a very low VSWR presented to the TX
on all bands. Even a used Johnson Matchbox (my favorite) is affordable and with
one wire antenna - you can cover 80 through 10 meters easily. Even though the
Antenna feedpoint impedance is high, so the VSWR on the openwire line is high,
it doesn’t matter - because the feedline loss is so small due to using ladder
line. This is not the case for high VSWR on coax fed antennas that are used on
multiple bands without traps or other techniques to keep the VSWR low.
When thinking about nulls in the antenna pattern, remember that most of these
antennas are about 50 feet high, so ground plays a significant part in those
nulls and they are very different from a free space model.
Using an openwire fed 80 meter dipole on 40 and 20 meters does result in gain
in many directions. This is one reason the openwire fed 80 meter dipole is my
favorite Field Day antenna. It can go on any band/mode (80 to 10) and is highly
efficient (with a good openwire tuner). It works on 15 and 10 meters just fine
too... and it is just one simple wire antenna...
73
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
john@kk9a.com
Sent: Monday, June 16, 2025 9:14 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] UL listed protector for ladder line
By resonant, I was referring to 1/2 wavelength dipole. I stated that a half
wavelength dipole will have a nicer pattern than using a W7FG ladder line
dipole across multiple bands. A half wave dipole pattern has two lobes and two
nulls, perhaps the nulls are -5dB. If you were to use a 100 foot long dipole
on 28MHz, beside the antenna having a very high feed point impedance, the
pattern would have a number of deep nulls, perhaps -15dB. The pattern and the
impedance of this 100 ft dipole would change dramatically, depending on
frequency. A fan dipole or the loaded dipole that K9YC suggested would have a
consistent pattern with no deep nulls and ~50 ohm impedance on the designed
bands. It would work well with the listed protection devices.
John KK9A
From: Ken WA8JXM <wa8jxm@gmail.com>
Sent: Monday, June 16, 2025 1:25 AM
To: john@kk9a.com
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] UL listed protector for ladder line
Resonance does not really change antenna performance. Feedline performance,
transmitter matching, yes. Antenna performance, no.You would have trouble
convincing me that a double extended zepp (!.28 wavelengths long), with more
than 3 dbd gain over a dipole performs worse than a dipole
Most AM broadcast antennas are not resonant. The most powerful AM broadcast
station ever in the U.S. originally operated with a 831' vertical , about 0.6
wavelength. It was later reduced to 747' and then 736', all non resonant
frequencies.
Ken WA8JXM
On Sun, Jun 15, 2025 at 9:36 PM <john@kk9a.com <mailto:john@kk9a.com> > wrote:
Besides it being easier to find a certified protector for 50 ohm coax, a
resonant dipole or a fan dipole fed with coax will have a nicer pattern and
perform better than the W7FG dipole with ladder line on non-resonant
frequencies.
John KK9A
Jim Brown K9YC wrote:
And, as noted, they won't work for your chosen antenna, for the reasons
noted. You should probably pursue one or more of the options for
antennas that are relatively well matched to coax on their operating
frequencies.
73, Jim K9YC
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