The Isotron is a real antenna, not a dummy load as some would suggest. Working with the Isotron 200B antenna is quite a learning experience however. If you remember my previous post, I was lamenting about the Isotron’s touchy tuning. First let me say it is a narrow band device and it can be quite unpredictable. I finally tuned the antenna in a horizontal position, with the 10’ mast hanging on two nails on posts of a walk out basement deck and the coil was approximately 5’ above ground. I got the antenna to tune to 1640 kHz @ 1.5:1SWR showing about 8 watts. The range was about 2 miles.But here is a rule of thumb to remember…when I raised the antenna to vertical position on my 30’ telescoping mast raised to only 11’ my resonant frequency was now raised to 1695 kHz…That’s about 55 kHz. The antenna is about 7’ from top plate to bottom plate and the bottom plate is now about 14’ above ground . The antenna included about 8” of tuning rod extension to the already 36” to tune at 1640 kHz (normal is 1650-1800.) The tuning rod was set to 23” over the top plate. The antenna’s tuning rod was then maxed out to about 32” above the top plate and the resonant frequency was lowered to about 1660 kHz @ 2.4:1 SWR showing about 4 watts. The total height is about 24’ from tip of tuning rod to ground. The range on my car radio was very good all things considered…2 miles very strong local, 5-6 mile distant, and about 8-10 miles fringe. That was yesterday…
It snowed overnight about 4” on the plates of the antenna and was cold this morning and my frequency was holding at 1660, but my SWR went down to 1.8:1, which is good thing. It’s going to warm up midweek and I’ll try to tweak the unit down to 1640 kHz by adding a small circular plate above the coil to raise capacitance. I’ll also try a few things to get a good impedance match to lower my SWR. Then I’ll raise my antenna another 5’ or so.
A word about antenna height, 25 to 30 feet gives you the biggest bang for the buck on frequencies below 2,000 kHz. You really wont get much more range for the trouble. On 100 MHz FM, it is a different story, the higher the better. That’s why high frequency FM antennas are put on a mountain, but low frequency AM antennas are put in a valley along river banks and salt marshes. The only reason that AM antennas are big is a matter of frequency and wavelength. The Isotron solves that issue. Another plus for low power AM is that you can get away with low cost RG-58/U which shows about 0.5 Db loss per 100’ below 2,000 kHz (The ARRL Antenna Book.). I use 50’ Radio Shack coax, and the losses are negligible. You’re better off focusing on resonance and impedance issues first if you want to get out.
I’ll keep you posted in Part II.
