[Amps] RF in the Audio
audioguy at q.com
Sun Sep 25 10:47:38 PDT 2011
I hate to step into this fray, but MFJ does. (no presumtion of quality, of course)
He Who Shall Remain Nameless also has a viable circuit on his website. It uses dual roller inductors; a "Balanced L" if I remember correctly.
Sent from my iPhone
On Sep 25, 2011, at 10:15 AM, "Fuqua, Bill L" <wlfuqu00 at uky.edu> wrote:
> Does anyone make a real balanced line antenna tuner? No toroidal transformer.
> If I built one I would couple link couple the TX into a tank and link couple the output with a swinging
> link with Fariday shield.
> You could not get any more balanced than that.
> Bill wa4lav
> From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf Of Carl [km1h at jeremy.mv.com]
> Sent: Sunday, September 25, 2011 12:14 PM
> To: Rob Atkinson; amps at contesting.com
> Subject: Re: [Amps] RF in the Audio
> The last place I would insert a balun is at any point in a OWL fed antenna.
> Do the twists as have been the norm since the 30's and live with whatever
> unbalance remains.
> Since OWL theses days assumes the use of a tuner then spend the bucks and
> buy/build a truly balanced one especially if running an amp. Compromises
> with QRP and barefoot can get away with a barely functional T200-2 iron
> powder 4:1 balun that comes with the low end tuners.
> I always use a LPF between rigs and amps and amps and coax feeds. A 12 large
> bead sleeve balun of 43 mix is at the input of each LPF and appears more
> than sufficient to keep RF inside the coax on any band. More beads are at
> every antenna feed point. All these conform to or are close to the 1000 Ohm
> impedance rule that has been a sort of ham standard for decades. Im also
> aware of the 1966 CIA document mentioned by K9YC on his site as I was
> Tempest cleared at the highest level at Sanders Associates 1969-78, a major
> DoD supplier and deep into stealth technology even back in the 60's. The CIA
> and other 3 letter agencies were regular visitors. I didnt remember the 5000
> Ohm recommendation however which showed some serious thinking that far back.
> After eliminating all RFI generators in the house using 2.4" 77 or 31 mix
> cores over several decades as more junk comes into the house Im assured that
> any digital crud heard on the radios is from external sources. The HRO-500
> on a 12V battery in the shack and an AM/SW portable as a sniffer have been
> used extensively.
> Remember also that the initial use of sleeve balun beads was due to TVI and
> when a dozen 1" beads could tame any tribander or trap vertical it was
> considered good enough. We didnt have PC's, switchers, digital everything in
> the house, etc, back then.
> Switching Beverages and changing directons of other antennas seems to
> confirm that the sources are thru the air.
> I have no use for OWL.
> Just last month a new and very loud noise showed up on 160; turns out one of
> the companies renting tower space changed to a new repeater and required
> several pounds of ferrite to tame.
> About 30 years ago I had a friend who owned a 2 way shop ask me to help him
> locate an IMD source that was driving him crazy at a repeater site. After I
> eliminated everything in the building....solid coax and other connections,
> no change in recordered VSWR's, etc we sat and studied the display on the
> service monitor. Remembering a USN experience from around 1962 I asked him
> to go outside and beat on the guy anchors and terminations with a tire iron
> from his van. That was the source of the problem, corrosion was causing
> diode joints and rectification of the RF. Back to the house for several
> dozen Snap-On chokes Id been stocking and selling for Yuri, VE3???, and they
> were put over the guys and taped in place. No more IMD. Later Yuri
> contracted with RatShak to stock them. Since the 160' tower belonged to the
> site owner we were not about to disconnect the guys and use the large beads!
> Ive since done that here to all 4 towers.
> While K9YC's site has a lot of very good information there is also a bit of
> disagreement with what others have published and I dont see that changing
> much in my lifetime. He hasnt bothered to reply to my request for a test of
> a balun feeding OWL fed dipole covering 160-10M and at 1500W. Lets try this
> with a 4:1 and 9:1 as those are the common ones in use as well as
> deliberately varying feed line lengths to present worse case scenarios on
> different bands.
> One test I rarely see mentioned is to test your coax first.
> Leave in place and terminate the far end in 50 or 75 Ohms and then tune the
> bands recording any crud frequencies. Then add a bead balun and reterminate.
> Record any differences in signal levels. If you have a quiet receiver then
> any pickup with the antenna is likely in "antenna mode" as I like to call
> it. Many get confused with all the technical terms used. Any additional crud
> picked up in "interference mode" will be small and easily eliminated at the
> shack end with another bunch of beads and hopefully a decent RF ground. This
> is no different than the Beverage coax procedure as has been in ON4UN's Low
> Band DXing and various web pages for awhile.
> The expense of DXE or other overpriced Beverage "boxes" is a waste of money
> unless you are incapable of following the well documented alternatives that
> can be tailored to individual requirements. A one size fits all box can have
> a wide range of performance in the real world.
> ----- Original Message -----
> From: "Rob Atkinson" <ranchorobbo at gmail.com>
> To: <amps at contesting.com>
> Sent: Sunday, September 25, 2011 8:27 AM
> Subject: Re: [Amps] RF in the Audio
>> The perceived problem with parallel balanced feedline has nothing to
>> do with the ability to achieve an acceptable balance in the system and
>> everything to do with the way most hams use the line and type of
>> matching network employed. Roughly 90% of so-called balanced tuners
>> are either non-symmetrical, inadequate in design or (this is the
>> closest to honesty) make no claim of being balanced but somehow claim
>> to handle balanced systems. The Johnson Matchboxes are genuine
>> balanced tuners that do the job right by putting RF currents in the
>> line that cancel and collapse the field. I've tested this with my
>> system using current meters and field strength measurements around and
>> in between my line in several random points. A better tuner is the
>> very hard to fine TMC TAC tuner, probably the best commercially
>> manufactured tuner ever made available to hams. But many hams express
>> dissatisfaction with the Matchboxes usually over their alleged limited
>> matching range. That gets us into a separate discussion about the
>> education of hams regarding tuners and their expectations, and is a
>> topic for TowerTalk.
>> The common mode problem exists where you have a balanced system, but a
>> noise point source is closer to one side of the system than the other,
>> so while you have equal and opposite transmit currents, you can have
>> c.m. on receive from a local point source near the antenna, such as
>> noise from a router or power supply in a neighboring home.
>> I don't think I'd employ DX Engineering as some sort of imprimatur or
>> validator for your work, as they are in the business of making and
>> selling products for hams.
>> Having read the rest of your email, I understand your points and your
>> statements are convincing, on paper at least, but such a choke as you
>> describe seems to be a solution to a problem that doesn't have to
>> exist, if an operator were to employ a method of impedance matching
>> and transfer from balanced feed to unbalanced that would allow for the
>> isolation of the balanced feed to prevent a complete common mode
>> I can see such a choke being worth a try for someone trying to force a
>> transfer with an unbalanced network, or with one of the symmetrical
>> tuners that contain a pair of synchronized roller inductors and a
>> single common capacitor. I operated with one of those for a few years
>> and did in fact experience c.m. issues such as conducted out of band
>> RF (a very strong electric service spark gap) detuning a vswr
>> analyzer, but in my case all these problems vanished once I started
>> isolating the balanced feed lines with inductive coupling (the
>> aforementioned Matchboxes). I believe that is a more robust and
>> reliable solution.
>> <<<MANY of the DX Engineering so-called baluns are common mode chokes --
>> indeed, what is commonly called a "current balun" IS a common mode
>> choke. Many DXE baluns that transform impedance are ARRAYS of common
>> mode chokes connected in series and parallel. If you open up some of
>> these you will clearly see chokes would not with coax, but with parallel
>> wires. And DXE DOES sell a common mode choke. I haven't bought one,
>> because I can rolll my own that are probably better for one-sixth of the
>> I HAVE inserted the bifilar chokes between the output of a Titan 425 and
>> the antenna tuner and tested at 1.5kW keydown for several minutes from
>> 1.8 MHz to 28MHz. At that point, the choke sees ONLY the differential
>> field, and there is VERY little heating because the field from one
>> conductor cancels the field from the other. Dissipation due to common
>> mode current is a very different matter, and is discussed at length in
>> the tutorial. In essence, if the choke as sufficiently choking high
>> impedance and the antenna is not very poorly balanced, the common mode
>> current, and thus the common mode dissipation, is reasonably small. If
>> conditions of the application (for example, impedance transformation)
>> place very high common mode voltage across a choke, the common mode
>> impedance must be much higher. In a testing situation, I have set up
>> very high common mode voltages and placed two chokes in series to
>> withstand them. DXE builds some of their impedance transforming arrays
>> of chokes that way.
>> As to mismatch -- a study of the fundamentals of transmission lines
>> would lead one to the conclusion that the loss due to mismatch in the
>> short length of 100 ohm line that comprises the choke is quite small.
>> After all, one of the most common uses of parallel wire line (notice
>> that I do NOT repeat the fiction of calling it a balanced line) is to
>> minimize the loss due to mismatch when feeding antennas that are wildly
>> mismatched, like the "one-size'fits-all" dipole that is nowhere near
>> resonance on most frequencies where it is used. Think about this --
>> we're connecting an antenna that could be anything from 5 ohms to 5,000
>> ohms, plus reactance, to a feedline that is, perhaps, 400 ohms. The
>> insertion of a 24 inch piece of 100 ohm line simply modifies (and not
>> very much) the impedance of the antenna as seen by the line. And, if
>> wound using #12 copper, as the chokes I have described are, the loss is
>> VERY VERY small, as confirmed by my tests.
>> Now, I'm a guy who plays by the rules, and shares my work FOR those who
>> play by the rules, and my testing is done at that power level, at duty
>> cycles consistent with serious contesting. Someone who wants to run more
>> than 1.5kW can design and test his own solutions. :)
>> 73, Jim Brown K9YC>>
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