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Author Topic: DID I Blow up My RF Ammeters?  (Read 36612 times)
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flintstone mop
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« on: November 27, 2009, 12:00:00 PM »

Hello out there in radio land.
I changed out my 450 ohm window line and 3/8 wave VEE (192 feet ant/125 feet feed) For a store-bought "True Ladder Line"
600 ohm ladder and 240 feet antenna. One continous wire that transisitons from ladder to antenna.
No matter how I connect my ammeters into the circuit, they no longer read RF current. They obviously are passing RF through but not indicating. They were 5 amp peak. They used to read 3 amps on the old system with 300 watts carrier.
I have read other postings about increasing the range. I wonder if I should pursue repairing. How can I check that the meter movements are still good?
I can't believe the changes I made to the antenna system would be that dramatic!
Although I have received a couple of emails on my Hotmail account asking what I am using for an antenna. I was on the air with this new ant. Thanksgiving eve with W1UX and K4KYV. Apparently there was some big change, that I'm unaware of without my ammeters.

Fred
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« Reply #1 on: November 27, 2009, 02:13:48 PM »

Hook it up to a variac, short out the circuit with the meter in series with the load (short)  and slowly bring up the variac. You need a clamp on amp meter to verify how much current is flowing in the circuit. Or you could hook it up to a 100 watt light bulb in series with the meter, the meter should read about one amp.
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« Reply #2 on: November 27, 2009, 02:18:05 PM »

It also works on DC if they are thermocouple type. Definitely put something in series with the meter to limit current to the full scale value. The wires inside are usually very thin.
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« Reply #3 on: November 27, 2009, 03:05:36 PM »

Fire them up with a AA dry cell.  Not a whole lot of poop, will show a couple amps or so.

Dennis
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k4kyv
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« Reply #4 on: November 27, 2009, 04:31:11 PM »

From my experience, if the thermocouple is blown, the meter will show up as an open circuit.

The only other possibility I can think of would be if there is a shunt across the thermocouple to increase its range.

I blew two 0-5 amp thermocouple meters before I discovered that running the Gates into an unloaded L-network produces too much circulating current for the thermocouple.  One time, it was an antenna relay malfunction; the second time it was operator error, forgetting to switch the antenna back in line after it had been disconnected during a T-storm.

I just shorted out the terminals to the dead meter and left it in line.
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w3jn
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« Reply #5 on: November 28, 2009, 01:45:12 AM »

I'm wondering how you came to the conclusion that the stuff is junk, and why it might have caused Fred's ammeters to burn out (if they did).
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flintstone mop
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« Reply #6 on: November 28, 2009, 10:31:57 AM »

I'm wondering how you came to the conclusion that the stuff is junk, and why it might have caused Fred's ammeters to burn out (if they did).

Don, the meters still have continuity. RF is passing through, but no meter readings.

Hi John,
I was wondering the same thing. Brian, I get a sense that you really hate this stuff. No issues here with QRM from the surrounding area affecting the RX.
It's just a bunch of wire constructed as the familiar 600 ohm ladder line and at the feed point this wire turns into an antenna. What a simple idea and without the worry of bad connections. It's pretty well constructed....time will tell with our Winter WX in Pa.
The antenna made a helluva improvement in my TX and RX signal. I have been very disappointed in the 450 ohm window line and dipole since we moved here. THere's more people cursing at the window line because of it's shortcommings.
The ammeters are a mystery. I won't know until I do more troubleshooting and disassemble them. They may now be at a voltage point, maybe there 's more than 5 amps going out to the antenna system, dunno.

I'll be delving into their demise later today...SAT...No money left to go any where. Black Fri took care of that!!

Fred
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Fred KC4MOP
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« Reply #7 on: November 28, 2009, 11:31:31 AM »

Quote
that line is crap

I guess the main thing I don't like about the stuff is the fact that it uses # 16 wire.

Perhaps at 120 volts its good for 10 amps but how much loss does it have particualy at ten meters?


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flintstone mop
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« Reply #8 on: November 28, 2009, 12:29:06 PM »

It's #16 26 strand wire. Light weight and nice to work with.

Discovery time: Brian may be onto something with impedance. The meters work fine. I dissected them and the thermocouple is good and connecting a AAA batt across the meters, they indicate a flow of current. So Pandora's box has been opened.
What I have is the familiar Simpson RF ammeter 5A full scale. Model 135R I do not have any idea of the specs for these.
But I'm still sticking by my story for the True Ladder Line
MY MFJ buddy now says that with the questionable 600 ohm ladder/antenna that I have is nearly 1:1 SWR and the Xs is 2. I may not know all of the antenna magic terms, but past experience shows me that single digit Xs is a good thing. It's series reactance??? Could be inductive capacitive? Dunno The manual for the MFJ 259 is not clear either.

Fred
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flintstone mop
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« Reply #9 on: November 28, 2009, 12:59:54 PM »

I guess the main thing I don't like about the stuff is the fact that it uses # 16 wire.

Perhaps at 120 volts its good for 10 amps but how much loss does it have particualy at ten meters?

I took an old HP bridge to mine and as far as I can tell the line appears to be much greater than 600-ohms when connected to the antenna and that seems to be where the main problem is.


So Brian, I assume you purchased just the ladder line and not the entire package?
And soldered/connected to your existing dipole of what ever wire you're using?

Fred
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« Reply #10 on: November 28, 2009, 01:54:52 PM »

We cannot reach the finish line if we don't recognize feedline impedance transformation in balanced feed doublet installations and what happens at the shack end with varying lengths and impedances of feedline.
[/DEU voice OFF]

Typical doublet/openwire line installations are VERY forgiving in terms of imbalance.

You can easily destroy an R.F. ammeter movement without opening the shunt. Ask me how I know.

Look for a reading on twice or half the frequency if you suspect being at a voltage node.
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« Reply #11 on: November 28, 2009, 03:03:26 PM »

For Brian and anyone else who has either elected to dislike W7FG ladder line (more on that in a moment) or simply desires a different Z and/or heavier gauge wire, you can easily make your own with schedule 80 1/4 inch PVC pipe, the wire of your choice, and UV resistant cable ties.  In a lot of places the regular schedule 40 PVC holds up for years outside but sched. 80 may be better in the sunbelt. 

With a bandsaw or fine tooth wood saw you take a fist of 10 foot sections and saw off a wad of spacers at the spacing of your choice.  Repeat until you have enough spacers.   Take your two wires  you are using for the feed and poke a cable tie end through one of the spacers.  At the other end loop the emerging cable tie around one of the wires and poke it back through the pvc spacer where it came out.  It will grab and hold the wire at the end of the spacer.  When the cable tie end comes out the other end loop it around wire number two and stick the tie through the little plastic click stop ratchet and pull it tight so it holds the second wire to its end of the spacer.  Clip off the excess cable tie.  You can do this in much less time than it took me to write it.  Photo showing short example below.

Now that the dislike for ladder line has once again been established and a method for rolling one's own has been given, perhaps we can move on to Fred's problem.

73

Rob
K5UJ

Credit for the hb ladder line trick is something I can't take, as much as I'd like to; that goes to George, K9TRQ.  I didn't even make the sample -- that was done by WB9DNZ (I did snap the photo though).


* IMG_0004.JPG (296.75 KB, 1280x1024 - viewed 892 times.)
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« Reply #12 on: November 28, 2009, 03:12:00 PM »

p.s.  The spacers used in the W7FG line are simply black plastic pipe of the type used for lawn in-ground sprinkling systems.  You can find it for sale at Lowe's in the section devoted to lawn irrigation DIY supplies.  The quarter inch pvc and cable ties are extremely light weight.  That trick actually works better with heavier gauge wire, so go wild, use no. 10 or 12  Cheesy
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« Reply #13 on: November 28, 2009, 03:39:48 PM »

p.s.  The spacers used in the W7FG line are simply black plastic pipe of the type used for lawn in-ground sprinkling systems.  You can find it for sale at Lowe's in the section devoted to lawn irrigation DIY supplies.  The quarter inch pvc and cable ties are extremely light weight.  That trick actually works better with heavier gauge wire, so go wild, use no. 10 or 12  Cheesy

Forget that, I'd use at least 8 gauge wire and glass or ceramic spacers Grin

I wish I had something small enough to be able to shove the power through 8 ga.  This 4/0 gauge 100 foot ladder line span is getting HEAVY, specially when the wind blows Smiley


8 gauge is what feeds the driver amp into the pumphouse.  8 gauge heliax, that is Smiley

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flintstone mop
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« Reply #14 on: November 28, 2009, 03:50:13 PM »

The glass and ceramic is going to turn the assy. into a very heavy monster that will come down when tested by MaNature. I did all this with No. 8 solid and the ceramics in my JN days, early 90's.wow! It hurt to fix it.
So W7FG shudda used solid copper? 600 ohm ladder (6 inch spacing) was the mainstay for many Ham ops. And some prefer even wider spacing from my Googling around.
I have gotten over not seeing the ammeters move, I shudda taken a field strength reading with the old system and then compare what I have up now. I was depending on the meters....oh well, more on air testing with folks who remember passed QSO's on the other Ant system.
I posted earlier that the meters are good. The impedance changed dramatically that they barely indicate now. I would have to get my wife to PTT and I watch for any movement.

Fred
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Fred KC4MOP
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« Reply #15 on: November 28, 2009, 05:58:08 PM »

Ok then, this thread must be done. What more could be said without being redundant??  I'll bet there must be someone who has something to say?

There must be a way to determine when a thread is dead?

Wait a minute, I have more to say but your going to have to wait till tomorrow Grin Grin
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« Reply #16 on: November 29, 2009, 12:41:35 AM »

put your meter in series with a 100  watt bulb an it should measure about 1 amp when the bulb is lit.You can test it at 60 Hz.. You cant't test 600 ohm line unless it is terminated in 600 ohms not an antenna.
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« Reply #17 on: November 29, 2009, 05:30:29 PM »

bull pucky
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K5UJ
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« Reply #18 on: November 29, 2009, 06:18:14 PM »

not to hijack this thread or anything but since we are talking about the maligned ladder line, which I have been happily using for five or six years, I finally got around to conducting some field strength measurements late this afternoon.  I found a clear freq. at 3700 and tx 20 watt carrier into the horiz. dipole at 50 feet with the flashbox tuned to 50 j0 then I walked around outside with a cheap diode driven meter box connected to a telescoping antenna 30 or 36 inches long.  out in the yard under the dipole I adjusted the meter to read almost zero with the antenna vertical. Horizontal and parallel to the dipole I get a tiny meter reading.  With it vertical I walked around.  I walked up to the balanced feeders.  No meter movement until the antenna is 12 inches away from the feeders then it starts to move up.   more signal on the meter when the whip is parallel to the feed and 12 inches away, not surprising since it is now in line with the fields. 

No significant reading with the meter unit and antenna along side one of my steel dipole support masts.  Over at the 65 foot 80 meter vertical significant presence of RF on that antenna but nothing on the 160 m. inverted L wire co-located with it and hanging down about 3 feet out from the 80 m. vertical.   Perhaps it is de-tuned by the feedpoint L network tuner for 1.885 MHz. 

I'd say not getting any field strength reading until the 3 foot whip is inside 12 inches from the ladderline is pretty good.

Rob
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« Reply #19 on: November 29, 2009, 10:06:29 PM »

Back to topic.
The RF ammeters are at a current node of zero.
Both of them.
You have a well balanced feed line/antenna combo.
heh, heh.
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w3jn
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« Reply #20 on: November 29, 2009, 11:42:38 PM »



In my case what I think it did was just turn my antenna into a full length 1/2 wave right at 1.6MC with no feedline. I've tried everything with the coupler, parallel and series tuning combined and the MF band interference just won’t go away. The antenna works perfect on 160 meters, but it’s not supposed to. I’m receiving so much junk origination from the MF region that I’m actually getting some of those strong MF station signal harmonics all the up to 10 meters and I’m serious, it’s that bad. It’s desensitizing all of my receivers because the junk is so strong and it's not image interference either.

The line is definitely greater than 600-ohms. It might just be that your antenna impedance is so high now that you’re just not getting any current to read with those meters. It might also explain why other people have had similar issues recently as well.

I have a new 100’ roll of the heavy #14 390-ohm stuff and I’m waiting for one more thing to arrive. I’m going to change out the entire antenna in the next few days so we will see what happens. I’m pretty sure it will solve the problem though.

Not sure that #14 wire over the 16 ga W7FG will improve things, because it seems to me you have an inadvertent P-N junction somewhere in the system acting as a mixer.  If you can't tune out the BC t-rash using the tooner, it's most likely intermodulation products you're seeing.   Can't be the fault of the feedline unless it's broken somewhere and semi-conducting, acting like a mixer.
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« Reply #21 on: November 30, 2009, 09:57:13 AM »

Quote
Its not image or intermodulation and there is nothing else inline or in the mix

It's gotta be coming from SOMEWHERE.  It's not an image.  It's not intermodulation.  Then what is it?  That's my point - something OUTSIDE is acting as a mixer and creating trash up to 10 meters.  No way the feedline alone is responsible for it, unless it's acting like a mixer.

I also fail to see a way that the feedline alone - whatever the impedance - can change the resonance of the antenna.  I am betting you have a break in the feedline or antenna somewhere and it's acting like a Zepp.  All my of experiences with W7FG have been great, but if you think this causing your antenna problems, by all means change it.
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« Reply #22 on: November 30, 2009, 11:28:54 AM »

Ok then, this thread must be done. What more could be said without being redundant??  I'll bet there must be someone who has something to say?

There must be a way to determine when a thread is dead?

Wait a minute, I have more to say but your going to have to wait till tomorrow Grin Grin


yeah Terry,
It seems we go around in circles and the antenna gurus we have on the forum have left the building. So much chatter about the plusses of ammeters and here we sit befuddled.
Brian, it sounds like the antenna you're describing has or had a self resonance of 1.6mc  instead of the desired 3.6mc with the 600ohm ladder line?
Let's try to forget who made it. I had to change the taps on my tuner with the new system and I'm getting great reports now. More operating time and various band condx will prove out what is happening here. I wish I did field strength measurements before and after.
 
And Jack your theory almost makes sense about the meters being at a 0 current point? I'll have to do some testing on other freqs and bands to see what the results are. I may have started a thread with half baked info....dunno.

I have performed the tests posted here and the meters are fine. I don't have any thing to substitute to prove any theories out.


FRED
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« Reply #23 on: November 30, 2009, 01:09:14 PM »

Mine pretty much confirmed my suspicion that the line is much greater than 600 ohms and that it made my antenna more resonate around 1.6MC instead of 3.6MC...

I don't disagree that some of what I'm hearing like on 10-meters is in fact intermodulation because some of it is, but a lot of it is also harmonically related to stuff below the BC band. What I'm saying is that because the antenna is now self resonate down around 1.6MC that everything is coming in too strong and overloading my receivers, thus causing some intermodulation which is desensitizing my receivers.

The surge impedance of the open wire won't significantly change the resonant frequency of the antenna/feeder system.  The only effect it will have will be on the SWR of the resonant feedline.  The total length of one feeder plus one leg of the antenna is what determines the resonant frequency of a balanced open-wire fed dipole.

The surge impedance of two-conductor open wire line can be calculated from the formulae and charts found in any ARRL handbook from the 30's to the present.  It is a function of wire spacing and wire size, with perhaps a very small influence from the dielectric materials used.

For a multi-band tuned feeder system, 600Ω is an optimum compromise, since the load impedance may vary from 50Ω or less to several thousand ohms depending on the band in use and whether the feedline is working into a high voltage point or a high current point.  On the fundamental half-wave frequency and odd harmonics, a dipole fed at the midpoint will be fed at a low impedance.  On even harmonic frequencies, it will be working into a very high impedance point.  With an end fed Zepp, OTOH, the feeders will always be working into a high impedance since the end of the antenna wire is always a high voltage/Hi-Z point.

400-600Ω is optimum for physical reasons.  It is extremely difficult to construct two-conductor open wire line at Zο much lower than 400Ω.  Either the wire size is very large, the spacing is extremely close, or both.  You  might be surprised at how little it changes the impedance of a given pair of wires to go from 2" spacing to 6" spacing.  To go very far above 600Ω requires very wide spacing to the point that the feedline becomes unwieldy, or else conductor size becomes so small that the resistive losses will be excessive.  I  don't think it matters at all what the surge impedance of a tuned feedline is if it is built to any of the practical dimensions in terms of wire size and spacing.   If you are planning to run the open wire as a  non-resonant (flat) line, then surge impedance is of utmost importance.

To build open wire line much below 400Ω, you need to go to 4-wire or higher.  4-wire line is constructed in rectangular fashion, with the conductors at opposite corners of the rectangle tied together.  Six-wire line can be constructed as a cage, with every other wire strapped together at each end so that a cross-sectional view would resemble a Star of David.

Somewhere round here I have an article that gives formulae for calculating the impedance of multi-wire line, but I haven't been able to find it.  I recall something like 16-conductor line to reach an impedance below 100Ω.  The biggest problem with multi wire >2 conductors is finding or constructing the spacers.  

For homebrew two-wire line, I would prefer fibreglass rods over plastic pipe for spacers.  Critters like bugs and small spiders have a habit of taking up residence inside the hollow tubing, and likely causing rf losses.  PVC itself is not much better as a dielectric than old fashioned wax impregnated wooden dowel rods like they used in the 30's.  A good and cheap source for solid fibreglass rods is electric fence posts sold at farmers supply stores.  Just wear gloves when cutting and filing the stuff to avoid very painful microscopic glass splinters in your fingers.

Open wire line can also be constructed in coax form, like the 4-wire line described above but with all 4 conductors strapped together, and a 5th wire suspended in the middle to serve as the central  conductor.  I believe WSM used or still uses that configuration for the transmission line to feed their Blaw-Knox diamond tower.

As far is the spurious intermod products from broadcast frequencies, make sure that all wire connections make firm contact.  Sometimes a defective insulator or even a branch or leaf in contact with the feeder or antenna wire can cause problems. Absolutely the worst case of intermod (and TVI) I ever had was when I built the end-fed Zepp in Houston and unthinkingly used zinc-plated steel wire to tie the copper conductors to the ends of the  spreaders.  It worked great until after the first rain, when the dissimilar metals corroded and formed a diode rectifier at the end of every spreader on the feedline.  I took it down and replaced the steel wire with small gauge copper and the problem was gone.
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flintstone mop
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« Reply #24 on: November 30, 2009, 02:03:07 PM »

It's #16 G 26 strand wire.
My configuration for the antenna is 125 feet of feedline and 240 feet antenna.
70 foot high VEE ends 40 feet high
Maybe these numbers help with what is happening in radio land.

Fred
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