[TowerTalk] RF Choke for 40M dipole

[Dave Sublette]

Good morning all,

Saturday, with N4TY on the tower and me on the ground,  we got my Rohn 55 topped off at 155 feet.  All is looking good.  I took a little time last night on the bench and put connectors on the 2 section RF choke using the “unknown coax” and two pairs of “unknown" Ferrite beads.  Using a very good quality dummy load, the assembly showed 1:1 SWR to over 100 MHz using my Comet CA-500 antenna analyzer.  

For the next test I carried it in to the shack and hooked it to my faithful Heath Cantenna and powered up the KX3/KXPA100.  I sent dahs for a long time with 100 watts going through the choke.  It didn’t get warm at all.  The reflected power was much less than .05 watt on the 5 watt scale of my Diamond SX-1000 wattmeter.

I’m thinking this thing is ready to put up as soon as I enclose it with a section of PVC pipe.

To review — I measured the common mode impedance at over 1500 ohms from 6-8 MHz on my MFJ 269.  Now I don’t know the mix on the ferrite cores, so there might be some loss there.  An email I received during the previous discussion claimed that type 43 was better than type 31 at 40m.  KF7P had an excellent set of data on his website that shows the type 31 is better.  I believe I will order some type 31 cores.  In the meantime, I’m comfortable with this present choke.

Is there anything else I should know before I put this thing up?  I won’t mind changing it.  Climbing is good exercise :-)

This has been a fun project and I feel I have learned/relearned some good things here.  Thanks for your participation.

73,

Dave, K4TO