I’m very pleased that my “Mod-Bob” antenna ended up being one that used “tuned feeders”. Well – therein lies the rub. I don’t really, really understand this antenna – because it seems to look like three phased verticals like a Bobtail Curtain – but does so on 30M as far as EZNec is concerned. But the feed is not at all a classic Bobtail feed – because I am not feeding against ground radials – which would be a voltage feed. Instead, my feed looks more like a dipole current feed.
Here are some simple, yet very easy to understand references to look at for a 1/2 wl dipole feed:
and a 1/4 wl vertical feed:
I’ve heard conflicting opinions regarding the efficiency of an “L” tuner circuit vs. the more common “T” circuit, but the Palstar BT-1500A is designed for open line feeds to the antenna. It has several switches and settings to allow for high and low impedance matching – and seems like I will be able to learn a lot from this antenna and project. The manual is very good – its an education in itself, but with my very trusty and well used Rig Experts AA-30 antenna analyzer and “field laptop” (an old Dell “beater” laptop that can go out in the yard with me), I will be able to try different settings on the tuner and measure what happens with the AA-30. I’ll learn right away if I need to run a 12v line out to this remotely installed tuner – or if its “default” switched circuits are good on all low bands.
I do know that the antenna is 50 ohms on 160M. What is interested is each “side” of the feed is 117 feet long. This is almost 1/2 wl on 160M – which is pretty cool for a 160M antenna that fits in a small suburban back yard. So, for starters, the impedance is LOW on 160M.
I know for a facty that on other bands the impedance will swing high – I suspect that on 80M it is especially – because the KAT-500 does not like the SWR much at all and I’m restricted to 200 watts or so there. Otherwise – on 160, 40 and 30M its fine.
The AA-30 will give the full story – and then, by switching in combinations at the antenna where the BT-1500A will be mounted – I can really get down to business and document this antenna from a feed point perspective.
The actual measurements plus EZnec plots will be useful – I like “data visualization” because its a way to take a complex set of variables and produce something a non EE person like myself can understand.
I build data warehouses for a living, and love to get to the “art of understanding” with “visualized data”.