RG-178 is the Teflon 50 Ohm equivalent of RG-174 and RG-179 is a 75 Ohm
version.
I have several thousand feet of RG-179 available I can cut to length pretty
cheap.
Carl
KM1H
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: "'TopBand'" <topband@contesting.com>
Sent: Thursday, June 21, 2012 10:40 AM
Subject: Re: Topband: Chokes for Beverages
> 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
>>
>>
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>>
>
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