Thanks Jim for the very informative information you provided on this topic. Additionally, you seem to stress that using cheap RG-6 foil for Beverage feedlines (apart from not being very durable over time) may be not so wise if I am serious about common mode rejection. I just acquired a 1000 foot roll of Belden of double shielded RG-59 with BNC connectors affixed to each end. This will give me some better feedlines when I do a general replacement in the fall. I also have a nice piece of 75 ohm triax (two independant shields) and at one time I believe by grounding both shields at one end and leaving the outer one float at the other I could create a Faraday common mode noise shield. When I tried to raise that balloon some years ago here the idea was summarily shot down and it was explained that what may work for AC hum in a TV plant has no guarantee of working at RF for noise on TB. So the expensive Triax just sits there in the corner.
73 Herb On 6/21/2012 10:40 AM, Jim Brown wrote: > On 6/21/2012 5:56 AM, Herb Schoenbohm wrote: >> Jim, >> >> Thanks for the information. My whole RX system is based on 75 ohm RG-6 >> but I have plenty of RG-59 but that still will be a bit crowded for >> more than 10 turns. Do you know the mini coax number for 75 ohm coax? > No, but I picked up a partial spool of some 75 ohm mini made by Belden > at a Chicago hamfest several years ago. It has a custom part number, not > a standard one. Check the online Belden catalog. > >> (I presume I can find the right connectors or adapters since I doubt >> if the mini-coax is compatible with the standard F or BNC connectors. >> ) I could use RCA with adapters but that is not my idea of a solid >> connection. > I agree, but I'm not thrilled with F either, although that's what I'm > using because that's what the DXE hardware came with. :) > >> The wire balun idea sounds interesting. Is the impedance of 50 ohms in >> the unit you described close enough for a 75 ohm system for RX only? > Of course. Think about it -- the transmission line going through the > choke is a very small fraction of a wavelength, and the impedance bump > is 1.5:1. Same for the 100 ohm option. > >> Right now I have a common mode blocker of toroid, then a ground >> connection, the another toroid. both have 7 turns each side of the >> ground rod which is about 20 feet from the shack. I have remove all >> similar devices at the Beverage Feed points after hearing I had them >> in the wrong place there. > My view is that having the highest practical resistive Z is most > important, and that 5K is sort of a minimum design objective. Two chokes > with a ground in between forms a three stage filter in the common mode > circuit -- two series elements with a shunt to ground. I didn't think > of it first, but it sounds right to me. > >> Another question: would there be any benefit in replacing the double >> toroid with a ground in between with a simple 1 to 1 ration (3 turns >> to three turns #20 enameled wire common mode blocking box with the >> standard binocular core 243 which I use for the other single wire >> Beverage feeds? > I haven't done any work on this, so I have nothing specific to offer. > The weak spot of a transformer in this application is capacitance > between windings. Remember -- a core is a dielectric, so you have both > capacitance through the core and capacitance between the windings > themselves. The weak spot of a choke is that the choking impedance isn't > high enough. Which is better? I don't know. What we're dealing with is > a very weak signal and potentially strong noise current, so more > isolation is better. :) > > There's another important element of this. Noise can be coupled from > the noise current on the shield of coax to the center conductor > proportional to the transfer impedance of the shield. The lower limit on > that impedance is the resistance of the shield at the frequency of > interest. The other element of transfer impedance is the uniformity and > density of the shield, often described as shielding effectiveness. > Foil/drain shields designed for use at VHF/UHF (i.e., CATV coax) have > fairly high resistance at 2 MHz as compared to something with a decent > copper braid, so they're not great shields at MF and low HF. One of the > reasons for using chokes on that coax is to kill that shield current to > prevent that noise transfer. > > 73, Jim K9YC > >> >> Thanks for the help on this >> >> >> Herb Schoenbohm, KV4FZ. >> >> >> >> >> >> On 6/20/2012 5:59 PM, Jim Brown wrote: >>> On 6/20/2012 1:16 PM, Herb Schoenbohm wrote: >>>> Jim, How about 7 turns of coax then an independent ground rod and on >>>> the other side another 7 turns on type 31 >>> Two chokes with a rod in between is a great move, but you need at least >>> 14 turns to get the choke resonance down to 160M, and more turns is >>> better. >>> >>>> Seven turns of RG6 on a 2.5 inch type 31 is about all I can get on a >>>> core.....or what about stacking cores? would that be any better? >>>> >>> Yes, it's OK to stack cores, but you still need a lot of turns. >>> Inductance increases linearly with the number of cores, but increases as >>> the square of the turns, capacitance increases linearly as turns but >>> only slightly with number of cores, so turns move the resonance more >>> than cores. >>> >>> Since it's an RX antenna, use smaller coax for the choke(s). RG58 or >>> RG59 or even mini-coax. A small break in the Zo doesn't matter. Or wind >>> one of the bifilar chokes (really a parallel wire line) shown in my RFI >>> tutorial.16 turns of a pair of #12 THHN will give you a 5Kohm choke from >>> 160M up to 15M with a Zo of about 100 ohms. Tape the wires together, >>> then wind them on the core(s) and put SO239s on each end. Do the same >>> thing with #12 or 14 enameled wire and you'll get about the same 5Kohms, >>> but not as broadband, and a Zo of about 50 ohms. >>> >>> 73, Jim K9YC >>> _______________________________________________ >>> UR RST IS ... ... ..9 QSB QSB - hw? BK >> >> ----- >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 2012.0.2179 / Virus Database: 2433/5064 - Release Date: 06/12/12 >> Internal Virus Database is out of date. >> >> > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK