Palstar BT1500A Schematic and My Switching Arrangement

I’m not exactly sure I understand how to read this schematic, but for all bands, the relay switching that I have going on is >50 ohms, LP or the UP position(Hi-Z) -which by default has the 65 pf engaged at the output – and then the Low C, where the larger 960 pf capacitor is taken out of the circuit. The resulting circuit looks like this:

Its pretty cool how the BT1500A uses two 40 amp contact relays to switch the capacitors either at the input side or output side of what is basically a balanced L circuit – that looks like a Pi circuit:

The inductors are uH . . .

As I was hoping, this tuner is PERFECT for this application. Its one of only a few times where this tuner being remote at the antenna is in fact, much better than a T tuner in the shack. It also explains why on 80M, the KAT-500 would match but I had to limit the drive so that no more than 200 watts was generated by the KPA-500. It would be fault city if I drove it more – so I knew the impedance at the antenna was probably quite high. On 160 and 30M I think the impedance is a lot lower – especially 160M, where the Mod Bob had a very low SWR even without the tuner – because it was basically a half wave dipole.

The other thing I’m learning is a bit more about antenna matching and the relationship of inductance and capacitance and reactance and resistance at the antenna feedpoint.

When you force a match, there will be losses, but to have a multi low band antenna that covers 160, 80 and 30M is no small feat. If I didn’t have a very good mono band antenna on 40M, this antenna would work fine on that band too.

I have a strong gut feeling that the stress that was on the KAT-500, or on any in shack tuner is not nearly as bad with this tuner at the feedpoint, nor do I think the current / balun will heat up as much. The KAT-500 is also an L tuner, but when I saw so many faults with the KAT-500 at that lower voltage, I knew at 1500 watts I needed a better solution, and I am VERY confident that I have made the right choice. At 500 watts and below, you can get away with a lot more, but I think its better to always design something that can handle high power, because you will know right away of your circuit or shack setup tries to run QRO and you have amplifier faults.

Now I am really feeling good about not just going out and throwing money at something like the AT-Auto. My gut feeling to try this “different” route is turning out to be a good thing.

 

2 Comments on “Palstar BT1500A Schematic and My Switching Arrangement

  1. This is the way to go. It’s so easy to have a autotuner in the shack but really the best is as close to the feedpoint as possible. Many amateurs forget that a tuner in the shack (or even in the radio) doesn’t solve the loss you have in your feedline. Especially important when 1 antenna is used for all bands. I use a autotuner at the feedpoint of my homemade multiband vertical since 2009 and think it is the best solution so far. This year I proved that even with my way to short antenna it is possible to have a QSO across the pond on 160m. Part of this is due to the low loss in the coax. The rest is the capability of JT65 digimode. 73, Bas

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    • I’m not entirely sure, but there may also be less stress on the current choke / Balun and tuner components.

      73,

      Rich
      KY6

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