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Author Topic: Blocking Capacitor for Ladder Line  (Read 60432 times)
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aa5wg
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« on: May 13, 2013, 02:05:57 PM »

Hi to all,

How do you determine blocking capacitor value and type that is placed inline on both sides of ladder line?

I have read values for blocking capacitors should be .002 pf, 4700pf door knob, mica types and .001mf of unknown type. 

Chuck
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Steve - K4HX
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« Reply #1 on: May 13, 2013, 08:19:56 PM »

Blocking capacitor? What is it blocking?

If you mean series tuning caps, wouldn't you want variable caps?
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KB2WIG
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« Reply #2 on: May 13, 2013, 08:41:17 PM »

I think its the final DC on the tank and off the antenner?


klc
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aa5wg
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« Reply #3 on: May 13, 2013, 10:07:23 PM »

Sorry for not being more clear on the blocking capacitor.

The blocking capacitor is a fixed value capacitor that connects the end of the ladder line to the the link coupler.  There will be one blocking capacitor for each side of the ladder line.

Blocking as in DC blocking is what I was looking for.

Chuck
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« Reply #4 on: May 13, 2013, 10:41:23 PM »

Why would there be any DC on your OWL?

Are you doing a remote relay setup or such?

73DG
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KB2WIG
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« Reply #5 on: May 13, 2013, 10:48:22 PM »

D,

"Why would there be any DC on your OWL?"

I think thats what he wants to prevent.


klc
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Steve - K4HX
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« Reply #6 on: May 13, 2013, 10:49:03 PM »

Where would the DC come from?
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K5UJ
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« Reply #7 on: May 13, 2013, 11:25:34 PM »


The blocking capacitor is a fixed value capacitor that connects the end of the ladder line to the the link coupler. 

If you have a link coupler you don't need a blocking cap.  the link will stop DC.  But most rigs, in fact all I have seen have blocking caps if they don't have a link tuned output network.

I suggest you start putting together a vintage reference library.  At a lot of hamfests, there are guys with boxes of books.  Go through them and buy any handbooks especially Bill Orr Radio Handbooks, ARRL, books by Terman, Sterling, NAVSHIPS basic Electronics, old first phone license manuals, just about anything from the 30s, 40s and 50s, and do some reading.  this includes old ham magazines too.   
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K1JJ
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« Reply #8 on: May 13, 2013, 11:29:19 PM »

Hi to all,

How do you determine blocking capacitor value and type that is placed inline on both sides of ladder line?

I have read values for blocking capacitors should be .002 pf, 4700pf door knob, mica types and .001mf of unknown type.  

Chuck

Chuck,

Yes, a blocking cap isn't needed, but to answer your original question in case you need it for another project...

One thing to consider is how low will the OWL impedance be at a coupling capacitor if the tuner happens to be matching a low impedance like when series tuned?   If we want the capacitive reactance to be at least 1/10th or less of the match impedance, then even .01 uf might not be big enuff.

At 3.8 Mhz, .001 uf = 41 ohms Xc.

.01 uf =  4.1 ohms Xc


So, if the match happens to be very low, like 15 ohms for a shortened dipole, the cap shud be about 1.5 ohms (or less) of Xc =  .03 uf.

If the match is always a high impedance like 400 ohms or more, then  .001 uf will be plenty.


This requirement will get even more demanding on 160M and diminish on freqs above 75M.


T


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« Reply #9 on: May 14, 2013, 07:10:43 AM »


This requirement will get even more demanding on 160M and diminish on freqs above 75M.


At that point if you are running some decent power (which will be needed on 160) your problem will not be the cap; it will be the inductor(s) current rating.  you'll need one rated for 10 amps, maybe more on 160 with power and a low Z like 10 ohms. 
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flintstone mop
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« Reply #10 on: May 17, 2013, 07:46:48 PM »

Maybe Chuck is afraid of the D.C. potential that a T-368 may put on an antenna when something craps out in its tank circuit.
Most transceivers whether SS or Tubes and even QRO amplifiers will never put D.C. on the antenna terminals.
Fred
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Steve - K4HX
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« Reply #11 on: May 17, 2013, 08:06:08 PM »

How would the DC make it through the link?
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KJ4OLL
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« Reply #12 on: May 18, 2013, 08:31:45 AM »

Hi Chuck,
Here is what I did, based on a design that AB5WG described in ER Number 284, Jan-2013.

2- 4700pf 5kv doorknob caps
2- MFJ P/N 10-15197 PLATE CHOKE, 1.8-30MHZ, 1.5A, 4KV, AME AMP
2- Gas discharge tubes, Mouser P/N 650-GTCA28-402M-R03
1- Plate of 3/16" copper for the ground. Not shown is the 30' copper strap that connects to the plate, and is buried
at the base of the PT 4x4 that this system mounts to.




73
Frank
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w1vtp
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« Reply #13 on: May 18, 2013, 12:32:38 PM »

Gotta admit that this thread has been a little confusing.  As has been already stated, There would not be a chance of DC out of the link coupled tuner unless the link accidently touched output coil and there was a failure in the tank of the amplifier.  Even then, the link might short out things unless the whole link got elevated above ground.  In any case I would think that design of the final should be such that that possibility would be more than remote.

Now having an inductive component on the OWL is another matter.  Would that not require some sort of transmitting high power mica cap or a couple of variable caps - either vacuum or breadslicer type?  Isn't the reactive component (albeit inductive or capacitive) on an OWL dependent on the antenna length / height above ground and the actual length of the OWL?

I haven't messed with OWL since I was a jn so have a lot of catching up to do
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Steve - K4HX
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« Reply #14 on: May 18, 2013, 01:10:22 PM »

It's all pretty confusing Al. The photos show series caps in the OWL. What is the purpose of these caps?

Any transmitter properly built will have a choke to ground on the output. In the event that DC from the final somehow makes it to the output, the choke would short the DC to ground and blow a fuse, pop a breaker or otherwise shut down the transmitter - DC gone.

The photos also show a large copper plate in close proximity to the OWL. This creates unwanted capacitance to ground. All other things being equal, the less stray capacitance to ground, the better. It's not clear why you would want to increase this capacitance.
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w1vtp
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« Reply #15 on: May 18, 2013, 01:22:16 PM »

It's all pretty confusing Al. The photos show series caps in the OWL. What is the purpose of these caps?

Any transmitter properly built will have a choke to ground on the output. In the event that DC from the final somehow makes it to the output, the choke would short the DC to ground and blow a fuse, pop a breaker or otherwise shut down the transmitter - DC gone.

The photos also show a large copper plate in close proximity to the OWL. This creates unwanted capacitance to ground. All other things being equal, the less stray capacitance to ground, the better. It's not clear why you would want to increase this capacitance.

Thanks for the sanity check, Steve.  I’m interested in any OWL / tuna discussion because I see one in my future. 

Al
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Steve - K4HX
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« Reply #16 on: May 18, 2013, 01:58:43 PM »

Maybe there are a good reasons for these additions to the system, but no one has articulated such.
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« Reply #17 on: May 18, 2013, 02:42:33 PM »

Maybe there are a good reasons for these additions to the system, but no one has articulated such.

FWIW WAM I once had a 124'center fed with 110 feet of balanced line. It worked FB on 3885 with only a BalUn on the shack end.

Same exact antenna and feedline worked on 7290 with the addition of a pair of 200 pF caps in series with each lead of the feedline at the shack end. It may have been 100 pF. One of those two.
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Steve - K4HX
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« Reply #18 on: May 18, 2013, 05:54:00 PM »

Sure that makes sense. I'm pretty sure you didn't call them blocking caps or put them in there to keep DC off your tuner.  Wink
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W2VW
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« Reply #19 on: May 19, 2013, 07:03:18 PM »

I didn't call them anything. Just hooked them up and got on 40.

Proper terminology has always been a problem for me so it's nice to see AMFOE coming to the rescue once again for someone else : )
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W0BTU
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« Reply #20 on: May 19, 2013, 07:21:40 PM »

The photos also show a large copper plate in close proximity to the OWL. This creates unwanted capacitance to ground. All other things being equal, the less stray capacitance to ground, the better. It's not clear why you would want to increase this capacitance.

That is good general advice. But in this case, how can the capacitance be more than about 5 pF? Would a few pF of capacitance to ground really matter that much? And I can't see this arrangement unbalancing the line, either.
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73 Mike 
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« Reply #21 on: May 19, 2013, 07:26:19 PM »

The photos also show a large copper plate in close proximity to the OWL. This creates unwanted capacitance to ground. All other things being equal, the less stray capacitance to ground, the better. It's not clear why you would want to increase this capacitance.

That is good general advice. But in this case, how can the capacitance be more than about 5 pF? Would a few pF of capacitance to ground really matter that much? And I can't see this arrangement unbalancing the line, either.

On the upper H.F. bands and near voltage feed it would make a lot of difference.
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Steve - K4HX
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« Reply #22 on: May 19, 2013, 08:16:15 PM »

It can reduce common mode impedance, thus potentially greater common mode currents and reduced balance.
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KM1H
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« Reply #23 on: May 19, 2013, 08:45:11 PM »

Quote
Most transceivers whether SS or Tubes and even QRO amplifiers will never put D.C. on the antenna terminals.
Fred

Famous last words!

There is also a decent chance the amps antenna choke is already open due to a close lightning hit at some point in the past. About 30-35% of the amps I get in for service have a blown choke and if the plate blocking cap lets loose there could be fireworks and expensive repairs.

Same for OWL and link coupling, especially if its a swinger; accidents do happen and a couple of caps is cheap insurance. The cap can be a transmitting mica or a RF rated ceramic.

Carl
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« Reply #24 on: May 19, 2013, 08:52:56 PM »

Quote
Most transceivers whether SS or Tubes and even QRO amplifiers will never put D.C. on the antenna terminals.
Fred

Famous last words!


Same for OWL and link coupling, especially if its a swinger; accidents do happen and a couple of caps is cheap insurance. The cap can be a transmitting mica or a RF rated ceramic.

Carl

If it's a swinger and not a series resonated deal it has a DC short on the 50 ohm side no?
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