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to rf choke or not

1iwilly

Sr. Member
Dec 7, 2008
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does anybody know the correct amount of turn for the 11-meter band to end up with a 1.1 feed
I'm picking up a 2 pill base amp and I know they don't run 100% clean I read that this type is more effective than the second pic on PVC pipe
 

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There's a chart on here somewhere. I couldn't find it right quick.
However if I remember correctly it's 5 turns at 4.5 inches
 
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A choke will not clean up a dirty signal from the amp, it will prevent R.F. going back over the coax.
Fr cleaning up a dirty signal from the amp you need a in line low pass filter to keep the harmonics from reaching the antenna.
And don't overdrive the amp to keep it under the 1 dB compression point.
 
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Willy

Ferritre chokes done correctly should be none reactive, coax chokes are reactive,

a coax choke if you are unlucky can resonate the outside braid of the coax causing a very low common mode impedance & big increas in common mode current,

look for the black line on G3TXQ's chart indicating the ferrite choke is none reactive on the frequency you are using.

you must use the same mix ferrite & same size torroid as Steve to get similar results.
 
does anybody know the correct amount of turn for the 11-meter band to end up with a 1.1 feed
I'm picking up a 2 pill base amp and I know they don't run 100% clean I read that this type is more effective than the second pic on PVC pipe

Just to be clear - a RF choke (CMC) should not have any effect on your SWR. It just prevents RF from travelling back down the shield of your coax and causing EMI/RFI. SWR is set by adjusting your antenna's radiator length and/or matching system.

Justme is right, you need a Low Pass filter to remove harmonics. Here is a good option for your power level:

https://www.ebay.com/itm/Palstar-Model-FL30-Low-Pass-Filter/133523814681

Make sure everything is well grounded to a common ground buss/bar. That includes radio, amp/s, watt/swr meter, low pass filter, power supply/s, etc.

To specifically answer your question, 10 turns of RG-400 (don't use any other type of coax) around a FT240-31 toroid will get you ~4400 ohms of resistance. More wraps does give you more resistance but it also lowers the effective frequency - you will actually have less resistance @ 27MHz. Some "experts" say you need at least 5K ohms for a CMC choke to be effective so, you can always wind up 2 in series (as shown takes about 5.5 feet with 4" pigtails on either side). RG-400 can handle 3KW @ 27MHz.

Attached is something I put together for my 10-11 meter setup (2 x 10 turns in series = ~8800ohms @ 27MHz) - attach at the antenna feed point.

You can get 12 feet of RG-400 for $16+ on ebay if you don't have any which should be enough to make up 2 RF Chokes even if you make them for the lower bands.
https://www.ebay.com/itm/12FT-RG400...able-19-Stranded-Inner-Conductor/264196085449

FT240-31 - Mouser and Newark sell them for ~$5 each
https://www.newark.com/fair-rite/2631803802/ferrite-core-cylindrical-119ohm/dp/43K4703
https://www.mouser.com/ProductDetai...MI8ZSIh9qB7AIVga_ICh27KQDXEAYYBSABEgLQSPD_BwE
 

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thanks for the replies ok i get it now i have the antenna grounded with its own 8ft ground rod then the radio grounded with its own separate 8ft ground rod will look into low pass filters
 
CMC on the coax braid is what causes vswr to change significantly with changes in coax length or moving the meter to different places in the feed-line,

if changing jumper length changes vswr that is a good indication you have common mode on the coax braid,

common mode impedance of mast and coax outer braid are seen in parallel with the antennas feed-point impedance,

a choke will effect vswr if you have cmc on the braid and the choke reduces cmc,

if you don't have cmc on the braid then the longer the coax the lower vswr will read at the radio due to loss in the coax.

Walt Maxwells "REFLECTIONS" series of articles explains it

https://www.totalreflexion.net/PDF Files/Another_Look_at_Reflections.pdf

Part 2 - Countdown for a JourneyFrom Mythology to Reality
 
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a choke will effect vswr if you have cmc on the braid and the choke reduces cmc,

if you don't have cmc on the braid then the longer the coax the lower vswr will read at the radio due to loss in the coax.

All the more reason to use a >5k ohm RF Choke. By eliminating CMC on the braid, you don't have VSWR problems due to CMC.

Maxwell's "string of beads" approach doesn't cut it - maybe for QRP but not QRO or the power level the OP plans to use - at least economics wise.
 
Dr_DX

i use a choke at the antenna from G3TXQ's chart & torroids further down the line again from Steves chart,

i made 2x W2DU chokes 40x 31mix beads on rg-393 & another with 40x 43mix beads on rg-393 for experimenting.

Why have i started typing dyslexic
 
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Shakespeare knew nothing of RF - only that "2 B: Or not...2 B" (Text'ing pun) only of the effort of the literal literary sense of choking such a property, in the physical realm, was yet to be discovered...

I was just out here to let this threads' participants, past, present and Future, that I'm enjoying the thread so far..

I didn't really see any need for any guidance except to show support in this thread for the process and conduct all whom have posted so far have done a wonderful job of.
 
I’ve made a few attempts at winding my own with RG8X on an FT240-31 according to the late gentleman’s chart. (But this Coax needs 3.0” turns to avoid core migration; not recommended to use it where tight confines exist).

Next up will be to try (8) turns of RG59 (1.2” diameter) on an FT240-61.

(Both Coax types because I already have them).

I’ve also bought and used several Palomar Engineers cube chokes. As well, a Morgan Systems 411cb Bandpass.

Sort of can’t have too many ferrites

(Truck driver tossing money in air, but they’ve worked/helped).

YMMV.

The Real Solution:
Don’t drive a big truck!

.
 
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Dr_DX Thanks for the ebay find i got it for $25.00+the ride


Just to be clear - a RF choke (CMC) should not have any effect on your SWR. It just prevents RF from travelling back down the shield of your coax and causing EMI/RFI. SWR is set by adjusting your antenna's radiator length and/or matching system.

Justme is right, you need a Low Pass filter to remove harmonics. Here is a good option for your power level:

https://www.ebay.com/itm/Palstar-Model-FL30-Low-Pass-Filter/133523814681

Make sure everything is well grounded to a common ground buss/bar. That includes radio, amp/s, watt/swr meter, low pass filter, power supply/s, etc.

To specifically answer your question, 10 turns of RG-400 (don't use any other type of coax) around a FT240-31 toroid will get you ~4400 ohms of resistance. More wraps does give you more resistance but it also lowers the effective frequency - you will actually have less resistance @ 27MHz. Some "experts" say you need at least 5K ohms for a CMC choke to be effective so, you can always wind up 2 in series (as shown takes about 5.5 feet with 4" pigtails on either side). RG-400 can handle 3KW @ 27MHz.

Attached is something I put together for my 10-11 meter setup (2 x 10 turns in series = ~8800ohms @ 27MHz) - attach at the antenna feed point.

You can get 12 feet of RG-400 for $16+ on ebay if you don't have any which should be enough to make up 2 RF Chokes even if you make them for the lower bands.
https://www.ebay.com/itm/12FT-RG400...able-19-Stranded-Inner-Conductor/264196085449

FT240-31 - Mouser and Newark sell them for ~$5 each
https://www.newark.com/fair-rite/2631803802/ferrite-core-cylindrical-119ohm/dp/43K4703
https://www.mouser.com/ProductDetail/Fair-Rite/2631803802?qs=P8bU7i9nNAWMk1EJQzshLg==&gclid=EAIaIQobChMI8ZSIh9qB7AIVga_ICh27KQDXEAYYBSABEgLQSPD_BwE
 
ok here is where i got the info the guy states that the right part number is =Fair-Rite part number 2631803802
here is the video link=
 

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