[BC] AM Loads

[Cowboy]

On Wednesday 25 March 2009 05:22 pm, Ronald J. Dot'o Sr. wrote:

>  The AM-1A TX output goes through 3/4" foam filled 50 ohm heliax to a 
>  rocker type antenna switch with the switch output heliax going to the 
>  ATU input a short distance away.  There is maybe 15' of heliax for the 
>  whole run from the TX output to the ATU input.

 Then, that 15 feet probably counts more as an inductor than 50 ohm cable.
 It's simply too short ( in terms of wavelength ) for the Z of the that cable
 to matter much in and of itself. The same would be true of any rocker
 switches and associated plumbing.

On Wednesday 25 March 2009 04:09 pm, Ronald J. Dot'o Sr. wrote:

>  I'm trying to reconcile why the BE likes the dummy load but doesn't 
>  like an ATU input of 50 J0.

 Because what the TX is seeing is *not* 50+j0 !!

On Wednesday 25 March 2009 04:35 pm, towers at mre.com wrote:
>  I sounds like there is a mismatch in the X. Keep in mind that the X side
>  will transform with cable length to something not 50 ohm R at the TX.

 OR, due to normal tolerance, the cable is not EXACTLY 50+j0 over its entire
 length, or *is* something slightly different over its entire length, in which
 case a true 50+j0 at the ATU *must* appear as something different at
 the cable input, unless the cable is exactly 180 electrical degrees long.

On Wednesday 25 March 2009 07:39 pm, Ronald J. Dot'o Sr. wrote:

>  OK, let me ask this question from a different angle.
>  
>  You've just taken over 
  //snip//
>  What would you do clean up that mess?

 Exactly what you did, temporarily until I could properly evaluate what's
 really going on, and fix it !

>  Another question that I have is will someone please explain "Rotation"
>  to me.  Other than the electronics that the AF taught me a lot of my
>  Broadcast experience has been OJT or self-taught and "Rotation" hasn't
>  been part of it.

 It comes from two places.
 If the load at the far end of a 50 ohm line is, for sake of argument, 50+j1
 then it will appear as 50-j1 1/4 wave away, and "rotate" back to 50+j1
 1/2 wave away.
 If it's 51+j1 at the far end, things get real complicated, because both the
 R and j will change as the Z is "rotated" by the line.
 If you plot the values on a Smith chart, they literally rotate around a
 center point. ( which may not be 50+j0 )

On Wednesday 25 March 2009 10:07 pm, Dave Dunsmoor wrote:

>  How do you guys who know this stuff well, adjust an ATU for not only low
>  reflected power at carrier, but also at +/- some amount of khz to effect a
>  clean match for good sideband response?

 Two ways.
 One, do the calculations at carrier, AND at both plus and minus some frequency
 difference from carrier. A "sweep" if you will.
 Then, select components that will show the proper response at all three ( and
 sometimes many more ) points.
 In some cases, this might mean you have two .0005 caps in parallel rather than
 a single .001 ( part of the black art parts ) or two .001 in series rather than a
 single .0005.

 It may also mean that a network is added in series with a specific reactance
 out-board of the real network with a specific curve to compliment the curve
 presented by the real network when that real network is optimized for all of
 the other constraints.

 Sometimes, you may NOT want 50+j0 because the off carrier response goes
 wacky, but you get real good sweeps if you make it 55-j13, and add another
 network with complimentary response to change that 55-j13 into 50+j0.
 If the site had an old tube rig, you may not have ( or want ) a 50+j0 input, because
 you get a much better bandwidth at the 55-j13, or maybe at 73-j8, so you simply
 licensed it at a non-50+j0 point, and tuned the rig to work with it.
 In these cases, you can REALLY screw up a station by making the "match"
 at 50+j0 !!

>  >
>  > I can't think of any other way to do it.  Common sense dictates that
>  > you tune the ATU to the best match and forget about all of the
>  > esoteric calculations and charts and just fix the damn thing!
>  >
>  > Ron D

 See the above paragraph.
 In most, but NOT ALL cases this would be a sane and rational approach.
 Those esoteric calculations and charts have their value, and that's why
 some of us are employed.   ;)

On Wednesday 25 March 2009 10:32 pm, Ronald J. Dot'o Sr. wrote:

>  First off I wouldn't say that I "know this stuff well", just enough to 
>  be dangerous.

 But asking questions WILL make you less dangerous, even if
 you don't like the answers.   ;)

>  I just tune the ATU for max performance (Min reflected) on the carrier 
>  frequency with no modulation and trust that the sidebands will take 
>  care of themselves.

 In the absence of resources ( whether equipment, experience, or
 the phone number of a sympathetic consultant ) so would I !!
 BUT I would do so knowing full well that the assumption may be
 quite wrong. 

>  Think about it, with two or three coils and a cap or two I think you 
>  would have a very hard time trying to narrow band it without a lot of 
>  other components.  Generally I would think that @ 1Mc the 10kc 
>  sidebands should cause minimal reactance, but that's just my 
>  uneducated opinion.

 Without doing a sweep, you can't know.
 Remember, the lower sidebands effect a greater change in frequency
 as a percentage of the octave than do the upper sidebands !
 The effect will be much greater at the low end of the dial, where 10kc
 is nearly 2% of carrier, than at the top of the dial, or even at the center
 where it's only 1% of carrier.

>  Common sense dictates that if you tune the ATU to the exact center 
>  that the sidebands should be equal.

 This a not uncommon, but faulty, assumption.

>  I have heard of some very narrow bandwidth towers causing problems but 
>  not being a PE I'm not sure of that.
>  
>  If you read the tower +/- 25kc either side of Fo and the readings 
>  progress the way they should then I don't see a problem.

 Provided the network is designed to compliment the shift, I'd agree.

>  I would think that if you read the ATU input +/- 25kc of Fo then that 
>  should tell you how even the sidebands will be for ATU and the tower.

 Most of the time, but not always.
 I have seen peaks and dips within that 25kc on some, but you need to
 do a real sweep to see it.

>  The biggest problem that I've encountered is getting the down time at 
>  the best time of day to read the tower and ATU.

 No argument there !

>  The above is just common sense nuts and bolts engineering.

 It is, and in many, perhaps most, cases will be "good enough."
 There are many ( not just some ) real world sites out there, where
 such "common sense" is just too limited to really know what's going
 on through those networks, and especially combinations of networks
 ( which DOES include the transmission lines, and their non-lumped 
 phase shifts ) to give a realistic view of what's really there, and why.

 As Phil has mentioned MANY times ( and others ) there are older
 directional arrays that simply can not be "adjusted" for IBOC in any
 form, and will need to be completely redesigned for these reasons
 among many others.

-- 
Cowboy