This is my choke.
I’ll make you one for £50…
2 Likes
I understand the potential need when QRO from the shack. But before I pay even £5 for one, I’ll like someone to convince me that a CMC choke is even necessary for 10W to a portable antenna with a decent VSWR. Hence my asking if there’s a noticeable difference with or without the choke [rather than merely following the perceived wisdom for having one].
1 Like
Yes, it can be. I only made that because I noticed an issue on 20m with my EFHW. It might be because my homebrew rig isn’t in a proper metal case. So I suggest you only use one if you find you need it.
So, Richard, have you tried without the choke? If so, what happened [that was different]?
P.S. I’ve never needed one running 10W to my linked dipoles & EFHWs nor when I swapped out the MPAS coax (with choke) for coax with no choke to that vertical.
I’m trying to find out if the problem is real or not when QRP portable.
I was getting a very rough sidetone while keying CW. I always use the choke now so haven’t checked recently if it’s still necessary. I think it probably depends on where the antenna, counterpoise and coax are in relation to my operating position. Perhaps next time I’ll try without it and see what happens.
Richard
That’s interesting, and suggests RF was getting back into the rig. What sort of power where you using? And did you have/use an ATU?
5W. EFHW via 49:1 transformer. No ATU.
That reminded me, the two professionally-made EFHWs I have (from LNR Precision) both have impedance matching units built in but don’t know if they also act as CMC chokes. I assume your EFHW was designed for 20m (and has a decent VSWR there) so that’s a bit strange.
49:1 seems a high ratio. I used a 9:1 UnUn with my end-fed (random length) long wire at home to get its impedance in range for my LDG Z11-Pro II ATU to cope.
My KX2 has a built-in ATU so I use it even for the bands the EFHW was design for (and it does a great job tuning the EFHW for other bands too).
Hello Andy,
The originally VSWR doesn’t affect the need of a CMC, but the absent of a CMC may affect the resulting VSWR.
A little bit back in this thread you see my two CMC (in the right part on top).
And like Richard says, the need of a CMC depends from the situation. Because of some problems in the past (5 W out) I’m using my two CMC preventive every time. I couldn’t leave the CMC at home. I don’t know the exact situation on the next summit. And on the summit I save the time for testing the need of CMC.
73, Ludwig
1 Like
That one’s only worth £25 because you haven’t crossed the coax over to the other side half way round. 
3 Likes
An end fed half wave has a very high impedance and hence requires a high ratio. The antenna is actually cut for 40m (i.e. it’s about 20m long) but it is still resonant on 20m (and should work on 15m too) so I can change band without moving.
VSWR and common mode currents are unrelated. The latter is caused by lack of balance and probably applies to any antenna, even a dipole, due to non symmetric surroundings.
Not had an issue with 10w but when I created a linked dipole to take higher power I had problems with random stuff happening on the FT-857 when operating at 100w. When I dropped it back to 50w there were no such issues. There was no 1:1 choke at the top like the SOTABeams Band Hopper 4 it was modelled on.
M.
1 Like
I use them on both my Dipoles, Verticals and The SuperAntenna. With the SA MP1C it makes the tuning a lot more consistent. The coax is not acting as a radial.
I use the Sotabeam one at £2.50. I would be more than happy to put one together for you at £49.99 
For 50MHz 3 + 2 turns of RG58 is recommended.
For HF 7 + 6 turns of RG174 is recommended.
Tried getting you again today Mark, I could hear people talking to you, but not you. One day we will get a good clear signal!
Martin
3 Likes
Are you starting a price war?
3 Likes
I use 1/4 wave vertical with two or three gnd plane wires. No coax carried. End feed vert with rig at base. Use a tree or FG pole (carbon pole with special handling to avoid lowering SWR). Sometimes use one of two traps for additional bands. Winding up the 4 or 5 16 ft wires is done on the thumb and baby finger in a figure 8 pattern for rapid deployment later. Wind time is about 1ft/sec (31cm)/sec. Or 64ft in a bit over a minute. I found winders broke easily or weighed too much. Coax was just too much of a burden.
Photo of rig and wound antenna/gnd plane:
1 Like
The Butterfly Coiling (or Lap coiling) method is widely used by climbers. For shorter lengths of cable you can just lap it over your hand.
Well. Back when I started SOTA I had a 5m pole, 10m RG-174 and some wire. I made an Inv-V dipole for 60m. I set this up (just) in my garden and trimmed for match using an MFJ259B. I had made and used dipoles before but this time decided to “do it right”. I was connecting coax (unbalanced) to a balanced antenna so I made a choke balun out of the RG-174 at the feed point of the dipole. 6 turns solenoid wound on a CD pancake. Antenna worked, much SOTA success. I added links and made 40m/60m dipole. It worked well, more SOTA success. Then I remade the antenna for 30/40/60 and it still worked.
After a few years (3-4) the feeder fatigued so I bought a new 10m length of RG-174, fitted the connectors and replaced the feeder. I didn’t make another choke balun and suddenly my antenna had a rubbish match on 30m. 60 and 40 were fine but 30 seemed to resonant on 9.4MHz or so. Investigations back home showed that coiling the feeder over a lossy piece of salty lard (my porky arm) the match improved. I remade a choke balun out of the new feeder and normal service was returned.
I replaced the 5m pole (after driving over the old one) and the antenna was now mounted higher up the pole resulting in the angle in the Inv-V being different. I couldn’t get acceptable match on 60/30m. I remade the pole mount so the Inv-V angle was the same as the old pole and the match was back to normal.
I replaced the cable again last year as the old one appeared intermittent and had some issues with matching even though I had put a choke balun in the feed. Still having the old choke balun, I made the new one physically the same (number turns length from start of feeder to start of windings etc.) and the match became as repeatable as before.
Do I need a choke balun? I don’t know. If I had a system with different feeder lengths and the original dipole lengths were dependent on the feeder, changing the feeder to one that was the same length (to a cm or so) would not make such a massive difference. The choke at the dipole feed is doing something.
I’m sure I could play with the dipole lengths without a choke balun and make it work. However, whatever the choke is doing, when it is physically the same the original choke I used, I have 2 link dipoles that are quite docile in deployment over varying European terrain. The 30/20/17 dipole and a similar balun with an 8m RG-316 feeder has been used in EA8 over entirely different terrain (where my upper-outer tunes very differently than in the EU) and again behaved the same as in GM.
So whatever I have done, the choke is needed at the dipole feedpoint. I can also point out that Victor GI4ONL has made assorted dipoles for use on a 4m mast with no concern about baluns or chokes etc. and his system works well, just look at his logs.
People who understand EM stuff better than I do can suggest what is happening here. All I know is by following some ‘cargo-cult’ antenna fabrication I have dipoles that work wherever I had tried them.
YMMV
2 Likes
Yes, for all or some bands, depending from the situation and especially the feeder length (see here, balun No 2 = CMC No 1).
73, Ludwig
1 Like
I have used it with my KX3. I did a comparison between the MPAS lite provided coax & choke vs RG174 with the N9SAB choke and there was no difference. Not used it without choke.
Used it with/without choke with the SuperAntenna. I found I got more consistent SWR but this could also have been caused by my counterpoise.
I could have made one myself, but had this one already and much more compact than I would ever be able to make myself.
Yeah, it’s probably good housekeeping. I’m going to ignore my previous [trouble free] experience over many years with my portable [linked] wire dipoles with no feedline choke. The MPAS Lite is my first ever vertical and I know verticals (are monopoles and) are unsymmetrical whatever one does with a wire counterpoise laid out on the summit ground, So, having the CMC choke in the supplied 15m RG58 feeder makes sense.
I’ve been a bit confused by [some of] the online literature about placing the CMC choke at the antenna feed point and just before the antenna’s impedance matching transformer. But if the manufacture does that, I guess it’s okay.
I’ve order a couple of toroids [from SOTAbeams, £2.50 each] and will make a 6+7 turns CMC at the PL259 [antenna feedpoint] end for each of the 5m and 10m RG174 cables I’ve getting to replace the supplied feeder.
1 Like