Esoteric question on tuner inputs and RF design - Page 3 - diyAudio
Go Back   Home > Forums > General Interest > Everything Else

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 14th April 2012, 11:11 PM   #21
diyAudio Member
 
Joe Roberts's Avatar
 
Join Date: Feb 2006
Location: VA
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.
  Reply With Quote
Old 15th April 2012, 12:25 AM   #22
diyAudio Member
 
Joe Roberts's Avatar
 
Join Date: Feb 2006
Location: VA
Incidentally, the guy who wrote that coax balun page, Brian Beezley K6STI, is one very very smart ham.

88108 MHz
  Reply With Quote
Old 15th April 2012, 12:56 AM   #23
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
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.
__________________
I may be barking up the wrong tree, but at least I'm barking!

Last edited by Conrad Hoffman; 15th April 2012 at 01:11 AM.
  Reply With Quote
Old 15th April 2012, 01:20 AM   #24
diyAudio Member
 
Joe Roberts's Avatar
 
Join Date: Feb 2006
Location: VA
Quote:
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.
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...
  Reply With Quote
Old 15th April 2012, 10:38 AM   #25
Mike B is offline Mike B  United Kingdom
diyAudio Member
 
Join Date: Jun 2011
Location: Oxfurdshy'r
Quote:
Originally Posted by Joe Roberts View Post
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.
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.
  Reply With Quote
Old 15th April 2012, 04:52 PM   #26
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
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.
__________________
I may be barking up the wrong tree, but at least I'm barking!
  Reply With Quote
Old 15th April 2012, 06:51 PM   #27
diyAudio Member
 
Joe Roberts's Avatar
 
Join Date: Feb 2006
Location: VA
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!
  Reply With Quote
Old 16th April 2012, 04:45 AM   #28
Lavcat is offline Lavcat  United States
diyAudio Member
 
Lavcat's Avatar
 
Join Date: Dec 2009
Location: New Jersey USA
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.
  Reply With Quote
Old 16th April 2012, 09:05 AM   #29
Mike B is offline Mike B  United Kingdom
diyAudio Member
 
Join Date: Jun 2011
Location: Oxfurdshy'r
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.
  Reply With Quote
Old 16th April 2012, 03:49 PM   #30
diyAudio Member
 
Joe Roberts's Avatar
 
Join Date: Feb 2006
Location: VA
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!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Esoteric p2 / Clock (question) XXX Digital Source 2 27th December 2010 06:24 AM
Larsholt fm tuner front end for JLH tuner design-Any still available? djsb Analogue Source 0 25th December 2008 04:46 PM
Hey, where did my aux/cd/tuner inputs go? Audiofirst Analogue Source 5 21st July 2007 10:27 AM
An esoteric noise question scott wurcer Analogue Source 3 4th March 2004 04:52 PM
Question about esoteric resistors: Tantalum, Riken Ohm, Holco, etc. Eric Solid State 1 22nd August 2001 06:52 PM


New To Site? Need Help?

All times are GMT. The time now is 06:10 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2