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to rf choke or not
- Thread starter 1iwilly
- Start date
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.
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.
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.
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.
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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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
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
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.
Just a Pun...
This is also to let you know someone does read these messages...
I'm liking what I'm reading so far...
Andy
That is the question,
it depends what your into, some folk like that kind of thing.
That is the question,
it depends what your into, some folk like that kind of thing.
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 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!
.
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!
.
- No one is chatting at the moment.
