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Common Mode Choke - HELP

Apr 17th, 04:54

VK4CCV

Joined: Aug 30th 2010, 01:09
Total Topics: 0
Total Posts: 0
G'Day Everyone,

OK, I am hoping someone can steer me in the right direction.
I have a FTdx-5000 which was the main RF generator.
I had a Common Mode Choke at the Antenna Feed-Point (Dipole).
I have never had issues with creating RFI and what RFI I had caused was easily snubbed out.

Now, I have purchased a new ACOM-1500, which is about to be delivered and guessing that with this extra power available I may increase the risk if causing problems.

The RF Patch Cable that I have made, which will go from the 5000 out to ACOM in, I have wound 17 turns of LMR195 onto a FT240-43.

I this a bit of overkill, or is this fine?

Pete VK4CCV
Apr 17th, 05:18

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
An RF choke at the feedpoint will not choke off common mode currents induced by asymmetry such as a feedline run parallel with the dipole. It is best to bring away the feedline at right angles to the dipole.
As the USA legal limit is 1500 watts, we often use Teflon RG-400 for high power RF chokes. LMR-195 is rated at 890 watts on 10 meters.
It ought to be just fine with the VK power limit of 400 watts and a low SWR.

Zak W1VT
ARRL Senior Lab Engineer.
Apr 17th, 22:42

VK4CCV

Joined: Aug 30th 2010, 01:09
Total Topics: 0
Total Posts: 0
Thanks Zak,

Yes, I have a run of 450 twin-leadrunning off at right angles to the actual Dipole itself. The Twin=Lead then goes into a 4:1 Balun then the 1:1 Common Mode sits behind that.
This then connects to a 25' of RG213 into the shack.

The only run of RG195 is to interconnect the FT5000 to the ACOM that has 17 turns on a FT240-43 Core and is about 3' long.

I am hoping this interconnection is OK and I have not gone overboard with the 17 turns.

Pete


Apr 18th, 05:17

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
17 turns should work just fine.

Zak W1VT
Apr 20th, 18:52

VK4CCV

Joined: Aug 30th 2010, 01:09
Total Topics: 0
Total Posts: 0
Hey Zak, To follow on from this, what would you recommend on the output side of the Amp?

There is about a 5' rn from the Amp, to the Floor (currently LMR195) that goes through a floor mounted Feed-Through (SO-258), then about 15' of 213 to the balun.

First, is the 5' of 195 sufficient to handle the max 1500 or do you recommend replacing this with 213?

What Choking would you employ?

Pete
Apr 21st, 07:21

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
No, LMR-195 isn't enough to handle that amp if you accidentally run high power. I'd use 17 turns of RG-400 Teflon coax on an FT-240-43 core for a balun. K9YC has other high power designs on the web if you needs something optimized for a particular need. You definitely need a higher power cable like RG-213.

You may want to go to QRZ.com if you want suggestions on where to best put RF chokes in an antenna system. Not only does that site have much more traffic, but they have image hosting. Other sites require you to find an image hosting service and let you link to that.

But, nobody really knows someone else's noise situation. There are just too many possible noise sources out there. It really comes down to experimentation and perhaps using Internet resources like Hamspots, PSK report, and the Reverse Beacon Network to see how your reception compares. FT8 can also be useful. If everyone is hearing the USA and you can't, that suggests a noise or antenna issue. Sometimes antenna polarization is the big factor. A good vertical installation may not do well on short skip, but may do exceptionally well on very long DX paths instead.

Zak W1VT
ARRL Senior Lab Engineer
Apr 22nd, 01:34

VK4CCV

Joined: Aug 30th 2010, 01:09
Total Topics: 0
Total Posts: 0
Thanks Zak,

Much appreciated for your assistance with this one mate.

Pete

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