Portable preamps for 6m 2m and 70cm – part 3

Hi all,

This is continuing on from Portable preamps for 6m, 2m and 70cm – Part 2.

I have updated some board designs for both the preamp and the sequencer.

Here’s a look at the preamp board:

Preamplifier board

And here is the sequencer/dual bias tee:

Preamplifier sequencer and dual bias tee

As stated in the previous post, this bias tee is designed to operate both of the FT-817 and FT-897 antenna ports. The caps exposed to full brunt RF are much more expensive than the X7R caps that can be found on ebay, but this is part of the cost and convenience of having something that will bias the coax and allow 100 watts SSB to pass. The caps are rated to 5 or so amps, so this imposes a SWR limit. It is on the low impedance side, but of course the actual current varies along the cable in the presence of SWR due to the standing waves. The FT-897 will reduce power in the presence of high SWR, but this board could have its ratings exceeded if driven with 3 or higher SWR at full power. Of course, I have also considered some margin for the inductors that will face high RF power as well.

Away from the high stakes, I can use cheaper (much cheaper) caps. The relay is also a little expensive, but if you wanted to order 1000 boards plus components, I’m sure that just about all the components will get much cheaper!

There is no need to actually use this board for two bias tees, by not installing two inductors one capacitor and the two BNC jacks associated with one channel. All the other components remain required.

The 4401s are a few cents, along with the resistors. Both boards are designed for a BNC through hole jack. It is still possible to run alternative connectors, but these would need to be case mounted – eg a UHF or N connector, with a short run to the holes on the board for where the BNC jack would go. The planned BNC jacks also provide for case mounting, so the board will be held to the case through the jacks.

The preamp board has less scope to use cheap components. The relays selected there must be rated for the RF current going through – the AXICOM HF3’s are rated to 60watts continuous RF power and 2 amps. Most of the caps and inductors again must face high power RF.

I’m in the process of ordering parts and getting some boards printed to build this thing and give it a spin. I also look forward to increasing my contacts next John Moyle field day.

73 de Wayne VK3WAM

One comment on “Portable preamps for 6m 2m and 70cm – part 3

  1. […] This is a continuation of Portable preamps for 6m 2m and 70cm – part 3. […]

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