Like audio guys, hams range from geniuses to folks who can't change the batteries in a flashlight. Hard to tell who is who on the web.
I maintain that a ferrite balun approach is better for the hobbyist. It is a simple, broadband, idiot proof method that yields a low Q, resistive j0 choke. Clip on a few cores and you're done.
Choke baluns depend on geometric factors, coax type/cable materials, and environmental factors (e.g. proximity to external conductors). Given that they are reactive devices, they can get tricky. There is more than one resonance and the reactive quantities can have undesirable effects in situ.
See this page for a technical, measurement based discussion on one potential pitfall of reactive choke baluns:
Common-mode chokes
Those nice traces on the coax balun link do not look at the whole picture.
This linked discussion point back to the fact, mentioned above re: measurements, that impedances vary at every point on an RF transmission line. Different length cables make for different conditions at the endpoints.
Antennas are complex antenna systems which include the whole coax cable.
Three or four ferrites, $10 and you're good to go. Want more choking Z...add more cores. Sounds like a no brainer to me.
It gets trickier due to ferrite losses when the transmission line is handling high power but this is not an issue on receive.
As always, "better" is a value judgement that is dependent on the goals at hand.
Also, I should note that foam dielectric cable is a bad choice for coax coils...the center conductor will migrate through the foam and mess up the geometry of the cable, screwing up its efficacy as a transmission line and as a choke balun. You want common mode rejection and this will ruin it. Try solid PE instead.
I maintain that a ferrite balun approach is better for the hobbyist. It is a simple, broadband, idiot proof method that yields a low Q, resistive j0 choke. Clip on a few cores and you're done.
Choke baluns depend on geometric factors, coax type/cable materials, and environmental factors (e.g. proximity to external conductors). Given that they are reactive devices, they can get tricky. There is more than one resonance and the reactive quantities can have undesirable effects in situ.
See this page for a technical, measurement based discussion on one potential pitfall of reactive choke baluns:
Common-mode chokes
Those nice traces on the coax balun link do not look at the whole picture.
This linked discussion point back to the fact, mentioned above re: measurements, that impedances vary at every point on an RF transmission line. Different length cables make for different conditions at the endpoints.
Antennas are complex antenna systems which include the whole coax cable.
Three or four ferrites, $10 and you're good to go. Want more choking Z...add more cores. Sounds like a no brainer to me.
It gets trickier due to ferrite losses when the transmission line is handling high power but this is not an issue on receive.
As always, "better" is a value judgement that is dependent on the goals at hand.
Also, I should note that foam dielectric cable is a bad choice for coax coils...the center conductor will migrate through the foam and mess up the geometry of the cable, screwing up its efficacy as a transmission line and as a choke balun. You want common mode rejection and this will ruin it. Try solid PE instead.
Incidentally, the guy who wrote that coax balun page, Brian Beezley K6STI, is one very very smart ham.
88–108 MHz
88–108 MHz
Today I built the Wide Band Vertical Omni on that site, using just a few turns through a big ferrite toroid for the balun/choke/thingie. I haven't compared it to a simple twin-lead dipole, but my subjective impression is that the thing is fantastic. I used 14 AWG enameled wire with thin wood spreaders at the top and bottom, with a small piece of plexi and some nylon hardware in the center. It's designed to be hung from the center of the top spreader. Next week I'll see if it helps my guy in the machine shop.
This is all very interesting stuff, and even though I've done some high frequency work, receiving antennas and input circuits have a lot of considerations I'm not up on.
Things I learned today- I have a roll of miniature coax and no idea what it is. I measured the impedance and its 44 ohms. Never heard of such a thing. Probably instrumentation or HV cable. Any ideas on what it would be good for? It's like white/silver RG-174 with a brown center conductor.
RG-6 from Home Depot has a shield that can't be soldered. It must be braided steel or something, but even acid flux wouldn't allow solder to wet it. Crimp only.
You can't buy twin lead at the home store anymore! They used to have a ton of it.
It takes ten times longer to build anything than it looks like at the beginning.
This is all very interesting stuff, and even though I've done some high frequency work, receiving antennas and input circuits have a lot of considerations I'm not up on.
Things I learned today- I have a roll of miniature coax and no idea what it is. I measured the impedance and its 44 ohms. Never heard of such a thing. Probably instrumentation or HV cable. Any ideas on what it would be good for? It's like white/silver RG-174 with a brown center conductor.
RG-6 from Home Depot has a shield that can't be soldered. It must be braided steel or something, but even acid flux wouldn't allow solder to wet it. Crimp only.
You can't buy twin lead at the home store anymore! They used to have a ton of it.
It takes ten times longer to build anything than it looks like at the beginning.
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Quite possibly shielded wire of indeterminate, i.e. uncontrolled, characteristic impedance. It is often called aircraft cable, probably because it is used in aircraft wiring harnesses. Teflon inner and outer insulation, stranded inner conductor, inner insulation layer is fairly thin compared to RG-spec coax? If so, maybe that's that's it!
RF cable usually has a fairly thick dielectric and relatively skinny center conductor. RG-spec Teflon (PTFE) RF cable usually has a clear outer jacket but I don't know if this is true in all cases.
Aluminum shields (stranded and or foil) are found in a lot of crimp-intended cables. This scares me because aluminum is a very reactive metal in dissimilar metal junctions but it is cheap. There is a lot of metal in braid so long cable runs would cost $$$...plus meth heads would be stealing the cable for scrap in many areas.
Good RG-6 can be quite fine cable and it can be found at a nice surplus price. Follow around the cable truck and see if they drop a piece!
Antennas and radio generally can get super complicated the deeper you dig but, fortunately, you can get away with murder and still have a good time and enjoy decent results.
Face it....almost anything beats one of those giveaway twinlead antennas tossed haphazardly behind the equipment rack!!
Wait..that's what I'm using at the moment! OOPS!
But I'm getting blasted with local sigs here so I don't have to get fancy to get by...
Good post Joe, all points noted & generally agreed with (for the hobbyist)
Re the migration of center core thru foam, again very good point.
The Webro WF100 I use has a recommended min bend radius of 65mm (2.55 inches)
My bauln is 73.3mm (2.88 inches) diameter, so its bent tighter than recommended (& for any other cable that I can find incidentally)
But my antenna lives in the attic so does not get hit with direct sunlight & the high core temperatures that causes much of this migration. Its something to watch out for.
Joe, that sounds exactly like my spool of wire. Teflon, but the center conductor is quite heavy, maybe 16 strands of .008" wire. I should measure the pF/ft. It would make very pretty internal wiring for a preamp or something, but probably a pretty horrible lead-in from an antenna.
On the coax baluns, I tried to make one using RG-174 and had a lot of trouble getting it to hit and hold a tuning. The beauty of RG-174 is you can coil it quite tightly. I bought the RG-6 but haven't tried it yet- it's incredibly stiff. They seem to coat the foil shield with something, so I don't know if galvanic action is an issue, but I was really hoping to solder to the braid for test and construction purposes. I need to shop around for something better.
On the coax baluns, I tried to make one using RG-174 and had a lot of trouble getting it to hit and hold a tuning. The beauty of RG-174 is you can coil it quite tightly. I bought the RG-6 but haven't tried it yet- it's incredibly stiff. They seem to coat the foil shield with something, so I don't know if galvanic action is an issue, but I was really hoping to solder to the braid for test and construction purposes. I need to shop around for something better.
Make up a pair of interconnects with that teflon cable and listen to it. This stuff sounds pretty good in general. I found some variation but all of it was OK.
Some, not all, of this cable has an extremely high grade braided shield--densely woven and nice soft/flexible silver plated copper. Top dollar stuff.
I have rolls of several gauges and use it for shielded wiring, such as runs to panel mounted volume controls etc.
Aircraft cable will most likely not be super low C but for the lengths involved in preamp wiring and in moderate impedance circuits, within reason.
No idea what that coating is on the cheap RG-6 foil shield. I'd presume the metal to metal junction issues have been worked out, at least for the good pro stuff, since there is about a zillion miles of 75 ohm cable in use.
It seems almost all connectors are crimped on these days. I don't know if cable installers even own soldering irons!
Some, not all, of this cable has an extremely high grade braided shield--densely woven and nice soft/flexible silver plated copper. Top dollar stuff.
I have rolls of several gauges and use it for shielded wiring, such as runs to panel mounted volume controls etc.
Aircraft cable will most likely not be super low C but for the lengths involved in preamp wiring and in moderate impedance circuits, within reason.
No idea what that coating is on the cheap RG-6 foil shield. I'd presume the metal to metal junction issues have been worked out, at least for the good pro stuff, since there is about a zillion miles of 75 ohm cable in use.
It seems almost all connectors are crimped on these days. I don't know if cable installers even own soldering irons!
Some years ago (quite a few) I made a Y antenna for the FM band and matching transformer. I have to admit it did not work as well for the station I was trying to receive as a Radio Shack vertical. It may have been a polarity thing. (Ham here for almost fifty years.)
Unfortunately nothing on FM anymore that I care to listen to.
Unfortunately nothing on FM anymore that I care to listen to.
I've done some digging in the coax bend radius thing & confusing & all over the place kinda sums it up.
Looking into various trade work practices
The "rule of thumb" (when nothing is specified) in both US & UK is to use the cable diameter x10 is the inside (ID) bend radius = x20 diameter.
In USA more specific is NASA (also Boeing) under the heading MANDATORY MINIMUM BEND RADIUS
"Coaxial cables shall not be bent below the minimum recommended inside bend radius (6 diameters for flexible, 2 diameters for semi-rigid and rigid)"
US Navy recommend x12 (some manuals quote x13)
Strange its double for Navy but I guess inside aircraft is less severe than up the mast head of a ship.
In UK we have a trade organisation call Confederation of Aerial Industries (CAI) & they have a bench marking scheme for antenna & cables
In the cable system they set pass/fail standards that include static & flex bend tests
For the "100" series cables (similar to RG-6) the mechanical specifications for the cable is stated as ........
Inner Conductor: To be copper*
Diameter of Inner*Conductor: 1.0 +/- 0.02mm
Outer Braid / Tape: No contact between dissimilar metals
External Diameter of Sheath: 6.55 +/- 0.3mm (for our US buddies = 0.2578 inch)
Bending Radius: 40mm (1.57)
Flexing Radius: 75mm (2.95)
So between the jigs & reels I am more comfortable with my 73mm diameter choke balun
The cable is independently type approved for 40mm radius (80mm diameter)
The NASA standard of diameter x6 = (6.55 x 6) x2 = 78.6mm diameter
Considering its located in a the attic .......... then consider the bends we all put into cables when fitting it into wall plate boxes ....... I will go check after 1 year.
Looking into various trade work practices
The "rule of thumb" (when nothing is specified) in both US & UK is to use the cable diameter x10 is the inside (ID) bend radius = x20 diameter.
In USA more specific is NASA (also Boeing) under the heading MANDATORY MINIMUM BEND RADIUS
"Coaxial cables shall not be bent below the minimum recommended inside bend radius (6 diameters for flexible, 2 diameters for semi-rigid and rigid)"
US Navy recommend x12 (some manuals quote x13)
Strange its double for Navy but I guess inside aircraft is less severe than up the mast head of a ship.
In UK we have a trade organisation call Confederation of Aerial Industries (CAI) & they have a bench marking scheme for antenna & cables
In the cable system they set pass/fail standards that include static & flex bend tests
For the "100" series cables (similar to RG-6) the mechanical specifications for the cable is stated as ........
Inner Conductor: To be copper*
Diameter of Inner*Conductor: 1.0 +/- 0.02mm
Outer Braid / Tape: No contact between dissimilar metals
External Diameter of Sheath: 6.55 +/- 0.3mm (for our US buddies = 0.2578 inch)
Bending Radius: 40mm (1.57)
Flexing Radius: 75mm (2.95)
So between the jigs & reels I am more comfortable with my 73mm diameter choke balun
The cable is independently type approved for 40mm radius (80mm diameter)
The NASA standard of diameter x6 = (6.55 x 6) x2 = 78.6mm diameter
Considering its located in a the attic .......... then consider the bends we all put into cables when fitting it into wall plate boxes ....... I will go check after 1 year.
I seems to me that making a single bend is a more challenging configuration than a coil. The corner of such a bend is a danger zone for kinking and collapse of the cable geometry --especially if the cable is not solidly fixed in place to avoid movement. (Perhaps that is part of the reason rigid cable has a tighter allowable bend radius)
A nice smooth coil does not impose such concentrated stress.
In any event, an FM antenna balun in the attic is no mission-critical, life or death scenario. You might lose the antenna pattern, but that is about it!
A nice smooth coil does not impose such concentrated stress.
In any event, an FM antenna balun in the attic is no mission-critical, life or death scenario. You might lose the antenna pattern, but that is about it!
My CGIRS (Compulsive Get It Right Syndrome) has forced a change of plan
This afternoon I made the same <1/4 wave length (690mm - 27.18inch) as a 2 turn (109.9mm - 4.33inch)
I rechecked the SRF & found an error in my previous 2 turn number.
The new 2 turn is 111MHz - about right for 88-108 - so I think I'm done.
This afternoon I made the same <1/4 wave length (690mm - 27.18inch) as a 2 turn (109.9mm - 4.33inch)
I rechecked the SRF & found an error in my previous 2 turn number.
The new 2 turn is 111MHz - about right for 88-108 - so I think I'm done.
Well, tonight I installed the new antenna in the shop. It pulls in signals just fine, but I quickly realized what the real problem is. There's a bank of florescent lights in the shop that just tear up the reception. I don't know if it's a bad ballast or if that's just par for the course. I'll bring in a portable radio and see if I can localize the problem to just a couple lights, or if it's all of them. It's going to be hard to move the antenna further away and I don't know if it's possible to filter this stuff. Hints and kinks?
Electronic ballasts are notorious and not easy to deal with, especially in a commercial setting that you don't own/control.
Some ballast designs are better than others. Potentially really nasty variable duty cycle PWM circuits are at the heart of these things so a unshielded and unfiltered one can be really bad for EMI.
MAYBE it is a defective ballast...got a portable radio with Aircraft band on it (VHF AM)?
Might be time for INTERNET RADIO....don't knock it till you tried it. No, it doesn't sound great but better than nothing. I listen to live streams of my favorite Austin, New Orleans, and Philly stations all the time. Not much DIY opportunity in it but there is music to be had.
Some ballast designs are better than others. Potentially really nasty variable duty cycle PWM circuits are at the heart of these things so a unshielded and unfiltered one can be really bad for EMI.
MAYBE it is a defective ballast...got a portable radio with Aircraft band on it (VHF AM)?
Might be time for INTERNET RADIO....don't knock it till you tried it. No, it doesn't sound great but better than nothing. I listen to live streams of my favorite Austin, New Orleans, and Philly stations all the time. Not much DIY opportunity in it but there is music to be had.
I don't listen to SW much anymore, but my Sony 2010 makes a great noise sniffer. BTW, I measured my "aircraft cable" and it has a capacitance of 67.7 pF/foot, and a velocity factor of (I think) 0.66, not that I think it has much RF application. It's Teflon and the dissipation factor is 0.00015 so it should sound decent.
VHF interference from fluorescent tubes is likely to come from the gas discharge itself, not the ballast. In essence, you have a very hot antenna radiating Johnson noise. Ballast noise mainly affects lower frequencies. Your incoming signal has been attenuated by the building structure. Try positioning your antenna near a large window on the side of the building which faces the transmitter.
At one time, UHF/microwave noise sources for receiver testing were made by installing a discharge lamp across a piece of waveguide.
At one time, UHF/microwave noise sources for receiver testing were made by installing a discharge lamp across a piece of waveguide.
You vastly overestimate my options in this situation!
That's interesting about fast noise sources. Yet another way to generate random numbers?
Just to close this out, I did some sniffing with the 2010 and was amazed at how bad things were in the shop. There's no area to place the antenna that offers any real advantage over where it is right now. Still, it works pretty well, and better than the twin lead dipole it replaces. Maybe someday we'll be able to run a lead outside, but for now this is it. The Pandora is a good idea, but a few people are already doing it and my guess is too many would slow down the network to the point where management would say no to everyone.
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