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
|