Isotron Antenna - 80 Meter Project

2018 - 4 - 3
[Sharing my learning experience]

80 meters in the size of a birdhouse!

Testing The Isotron Photo

The following is from direct measurements and trials I have conducted.

Prior to the gable end roof mounting I did testing and tuning.
Testing was done using a 15 foot thick wall Aluminum pipe in my backyard and three ropes.
I had 124 feet of coax on the first test from the antenna to my transmitter, a Yaesu FT-891 with an FC-50 tuner.

By kicking the base out I could take it down and then raise it again quickly with out altering the ropes.
Resonance was very sharp with about 55 khz peaking bandwidth, and it had good noise immunity.
With the 3 to 1 tuner I am getting about 150 khz workable bandwidth for each configuration of the system.

Physical assembly is different for 6 sections across the 75 - 80 meter band
Within each "band configuration range", tuning bars are repositioned to move across it.
To work out of the present range, the antenna must be pulled down and adjusted again and/or reconfigured.
Photos above showing the 3.920 mhz configuration we would use up here in Alaska for ARES nets.
This was exactly what I was looking for in a very small antenna for the 80 meter band!


This photo showing the assembly I ended up with.
It is important to orient yourself to the antennas directions. The front of the antenna is the side away from the mast.
The rear is the side towards the mast.

Assembly Photo

Puzzling over the instruction sheets for the Isotron for several hours I still ended up assembling it incorrectly on the first trial.

What was missing on my end was a general comprehension that there are 6 different ways to configure it,
depending on the section of the 80 meter band you want to be able to tune by moving the bars
There are two tuning bars and two hats that can be placed onto them in 6 various ways.
I incorrectly assumed what you see on the left photo below.

Photo to the left below shows the lowest frequency configuration 3.5 - 3.675 mhz,
 however the bars are mounted on the rear side holes incorrectly and should have been on the front side holes.

Photo to the right below shows the highest frequency configuration 3.945 - 4.0 mhz. All tuning bars removed.
The part of the band I wanted to use 3.920 MHz only requires one of the bars be mounted to the top plate near the center.
I got the top plate mounted backwards, such that the upper tuning rod hole was on the rear side, rather then on the front side.
For me this was not self evident in step one [sheet 2] of the assembly sheet, although it is shown in one of the diagrams on [sheet 7].
I discovered this when I went to configure it for that range of frequencies when I got to [sheet 5] of the instructions.
Once discovered this was very easy to fix by reversing the top plate shown below.

Correct config photo

Within the "configuration range" you set up the tuning bars and hats for, we now change the "angle" of the tunning bar(s) to move the tight resonance through that small area of the 80 meter band.
The stubs are turned towards the front side of the antenna system, away from the mast to change the resonance point and move the tight resonance peak up that configured range.
For me, as I moved the bar shown below forwards the frequency of the dip moved higher. I ended up with it almost straight out to the front.

Tuned Isotron Photo

Tuning was done using an MFJ-259C. At the perfect resonant point the SWR was 1.3 to 1.
On the 15 foot mast, the resonance peak would move 80 khz lower in frequency on the ground then in the air.
Once this was charted, we then tuned it on the ground to 3.840 and when raised it came out perfect on 3.920 mhz.
Final setup shown below.

Final Tuning Photo

Photo above showing the front view for the final tuning at 3.920 - 3.933 mhz, now resonant perfectly on target.
With this understanding, it is possible to place resonance anywhere across the 75 - 80 meter band, but one must be sure to save all the extra parts!

I will be needing these again if I decide to set up for Winlink operations down the road around 3.583 to 3.595 mhz, see photo below.

Unused Parts Photo

Credits and References

Manuals are available online, as well as ordering information:

Bilal Company - Isotron Antennas For Over 30 Years

Document preparation:  David Lowrance AL5X

[For the betterment of Amateur Radio, this document is public domain.
Amateurs may use any of the material as you choose to, including my photos and all text.]