Link Coupled Antenna Tuner

[w5hro]

I've been looking over my homebrew link coupled antenna tuner to possibly make some changes. I drew up the attached pic of my current circuit which seems to work fine.

The main question is about the ground connection at the split butterfly cap C2. Would it be better to leave it grounded or floating and why?

My feedline is 600-ohm open wire line and the butterfly cap came out of a Johnson Kilowatt Matchbox.

I’m also going to add a BNC jack on the back (maybe the front) for a sample output to the monitor scope.

P.S This is also a test. There is one small thing wrong with part of the circuit.

Hint: Metering

[k4kyv]

Leave it floating.  Reason?  If you ground it, that will likely set up common mode currents in the feedline, so that working against ground, the feedline acts like a Marconi antenna with the flat top acting like a capacity hat.  This will show up as unbalanced  currents in the feeders and result in wasted power  due to ground losses, unless the ground connection is an extensive radial system.  It will also alter the antenna pattern due to radiation from the  feedline, but I would say that would probably be of little consequence to the radiated signal, although it might exacerbate local RFI problems.  The main problem with the phantom Marconi would be the ground losses.

[k4kyv]

The Triplet Thermo-Couple HF Ammeter works great too. Better than any lousy SWR meter setup

I have used thermocouple rf ammeters as my output indicators since day one.  They are true rms current reading devices, unlike rectifier type meters that read average current with the scale bogusly calibrated in rms values and which give correct readings only with pure single tone sine waves.

I do use a SWR meter on my 160m setup to adjust the L-network in the shack so that the Gates sees minimum reflected power.  That transmitter is almost as picky as a solid state ricebox about the load it works into. The reflected power null feature works well as a relative indicator, but the output power function on the damn thing reads more rf output than I am running DC input. 

Or maybe the meter is reading correctly and I have inadvertently run onto a solution to the world's energy crisis. 

[W2PFY]

Well, it’s obvious that no one has a clue so I have attached my correct HB circuit below.

Maybe some people don't care

You said your hint was the metering. What's wrong with it. I also notice in one drawing C1 is present and in the other it is absent. In the case of the BC-610 it is necessary to have a C1 either in the transmitter or at the antenna tuner because sometime it's not possible to decouple the link to get the plate current down to safe levels. Some people put two  meters in the output. That leads to a question since I have never done it. How are both meters to read? Exactly the same? If they don't read exactly the same, what is the problem?

Lastly, this type of RF meter can also be used at audio frequencies to test modulation transformers for measuring power or what ever your looking.They would be in series with the proper power and load requirement resistors for a given transformer secondary.

Caution is highly suggested since there will be very high voltages present. I would also suggest the meter be floating because of the forgoing statement.    

[W2PFY]

The loading caps (circuit) in your transmitter will usually take care of the rest.

  BC-610's do not have a loading cap.

Caution is highly suggested since there will be very high voltages present. I would also suggest the meter be floating because of the forgoing statement. 

This is suggested when using as a meter to measure modulation transformer power.

I would still like to know more about using them in the output side of the antenna tuner.

[flintstone mop]

Well, it’s obvious that no one has a clue so I have attached my correct HB circuit below.

That leads to a question since I have never done it. How are both meters to read? Exactly the same? If they don't read exactly the same, what is the problem?

    

I assume you are talking about two RF ammeters on the legs going towards the antenna feedline. A serious imbalance would mean that the antennna is not the same length at each end or something wrong in the feedline. A balance can be maintained by varying the taps on the coil up one tap or down.

Fred

[W2PFY]

RF ammeters on the legs going towards the antenna feedline

That's what I was axing about Fred, Tnx OM.  I'm going to be building a tuner this winter that will be rated for 3 KW or more mainly because that's the size of components I have. I'll be using two meters in the output. I think they are about 5 amps each and are a matched pair. Question, I'm assuming the current would be less on the output than the input by perhaps 1/2 ?

My memory is not that all that great on the BC610; does it have a low impedance 50-ohm output?

It's link coupled and it can run into 50 ohm coax but I think by grounding one side of the link it makes it 75 ohms so it must be a 300 ohm balanced output?

[K5UJ]

About whether or not to ground the post between the two caps on the output of the tuner, I'd conduct a simple test. Get on some dead part of 80 m. in the daytime (or whatever band you want to use it on) and have another ham 5 to 10 miles away on the same frequency looking at his rx signal meter.  Tx about 100 w. carrier unmodulated except cw ID, with the cap one way, then flip the ground switch and tx again.  Which ever way gives the stronger signal, that's ur way to go. 

Forget testing at night on skywave--too unstable--but a local ham close enough to be getting whatever ground or direct wave you have but not close enough to be in the near field will be able to tell you which is causing you to get more RF out, i.e. which way is more efficient.  If there are no local hams then put a rx, it can be any rx as long as it has a decent sig. meter, in ur car with some kind of pickup ant. on it and have someone, xyl maybe, drive out a couple miles and report back via cell phone.  Set it up so all she has to do is look at meter when you tx.  Then look at it again and tell you which is stronger. 

 

[W2VW]

A forced common mode on the feedline condition would make the signal much stronger 5-10 miles away during the day.

[k4kyv]

The BC-610 has a variable link coupled output.  Usually some type of tuner is needed between the link and the antenna transmission line.  The stock military antenna tuning unit is designed to match the 610 to a random length wire, not to a balanced feed line.

To match it to a balanced open wire tuned feeder, a link coupled tuner would be the best bet.

The 610 was not designed to work directly into a 50Ω unbalanced coax line.

[W2PFY]

The BC-610 has a variable link coupled output.

That said: Can any of you RF brainwaves answer this?

I'll be using two meters in the output. I think they are about 5 amps each and are a matched pair. Question, I'm assuming the current would be less on the output than the input by perhaps 1/2 ? To extend the thought, since this is a impedance matching devise and is a step up transformer it would seem to me that current would be 1/2 the primary. But then there are Two meters, perhaps the answer is more complex. Maybe I should read a book on it. Problem is, I don't have a book here on the subject. Maybe I could Google it?     

 

[ka3zlr]

About whether or not to ground the post between the two caps on the output of the tuner, I'd conduct a simple test. Get on some dead part of 80 m. in the daytime (or whatever band you want to use it on) and have another ham 5 to 10 miles away on the same frequency looking at his rx signal meter.  Tx about 100 w. carrier unmodulated except cw ID, with the cap one way, then flip the ground switch and tx again.  Which ever way gives the stronger signal, that's ur way to go.  

Forget testing at night on skywave--too unstable--but a local ham close enough to be getting whatever ground or direct wave you have but not close enough to be in the near field will be able to tell you which is causing you to get more RF out, i.e. which way is more efficient.  If there are no local hams then put a rx, it can be any rx as long as it has a decent sig. meter, in ur car with some kind of pickup ant. on it and have someone, xyl maybe, drive out a couple miles and report back via cell phone.  Set it up so all she has to do is look at meter when you tx.  Then look at it again and tell you which is stronger.  

A forced common mode on the feedline condition would make the signal much stronger 5-10 miles away during the day.

I think I'd rather have a slightly less signal during the day than have possible TVI/RFI problems around the house and neighborhood because the feedline is radiating. Reminds me of that crazy G5RV antenna design which is nothing more than an interference-generating radiator.

Na Aaaah say it ain't so, all those ham fest fill in prizes are RFI generators....

73
Jack.

[W2VW]

Terry, The current on the output terminals of your tuner will depend on the impedance of the load.

5 amps will not be enough on some bands.

When you have some reactance and/or a high impedance load the current can be quite low.

[W2PFY]

Gee, I don't get that circuit. It looks like a way to burn out your meters. What I do with BC-610's is put a fairly wide space variable cap in series with one side of the link to chassis ground. You can then leave the link fully coupled and use the added cap as a loading cap. It works fine and I would recommend its use if you do not have a link coupled antenna tuner for loading a coax fed dipole. You will need a cap that is about 1000 pf or more if you want to use this scheme  on  160 meters. to work it on 160 I merely shorted out the cap and it worked.  

[W2VW]

Several mil designs usually include a way to short the R.F. ammeters when not reading said meters.

[flintstone mop]

On my system I see 3 amps of antennna current running legal limit. My wire is untuned 180 feet inverted VEE, 70 feet high, ends are 40 feet high. THe antenna current depends on many things. If your dipole or VEE is lower to the ground there could be many more RF amps.

Fred

[KF1Z]

On my system I see 3 amps of antennna current running legal limit. M

Define "legal limit"

Contrary to popular belief, if it’s only 375 watts, then it’s too low. You can actually run more than that and still be perfectly legal.

There is still a real misconception among hams as to what is actually allowed.

Should be simple enough...

1500 watts PEP.
Carrier power has nothing to do with it.

[W2VW]

On my system I see 3 amps of antennna current running legal limit. My wire is untuned 180 feet inverted VEE, 70 feet high, ends are 40 feet high. THe antenna current depends on many things. If your dipole or VEE is lower to the ground there could be many more RF amps.

Fred

On my old antenna I got up to 18 amps to the feeders with a 1500 watt test carrier on 160.

[w3jn]

Example: My HB rig puts out about 400W into the 50-ohm input side which is only about 3A max.

Several mil designs usually include a way to short the R.F. ammeters when not reading said meters.

Yeah, that would be an easy thing to add. Ground to measure then short across when not grounded. Just use one ceramic rotary switch with enough poles and connections.

Look at the attached. SW2 is not in the right switched state, but you can see the idea. Short across and disconnect the meters when ungrounded then connect and ground the meters to take the measurements. Easy as pie..

I guess I don't understand the point of this circuit.  It's kind of like hooking an ammeter directly across the output of a power supply.  Doesn't really tell you much of anything other than how much smoke the ammeter contains 

Why would you want to connect an ammeter between the output of a tuner and ground?  Ammeters oughta be in series with the circuit being measured - in this case, the antenna.  If you shunt all the RF energy to ground thru the ammeter, what effect would the antenna have on the RF current reading?

[w3jn]

I did read your last post - I read every post in the thread, but I still don't know what your strategy is here.  Dave VW mentioned he's read 18 amps in series, and you responded that you don't like that idea for that reason. 

If you don't put them in series with what you're trying to measure (the balance of the antenna and feeders) I don't know how you can measure it.  The RF ammeters are essentially shunting all your energy to ground and are only measuring the ability of the transmitter to supply current. 

Are you trying, at some relatively low transmitter output, to measure the inherent balance of the tuner itself, or find the taps on the coil where it's balanced?  That's fine if the ant/feeders are 100% balanced but very seldom will they be, due to surrounding objects, etc.  That's why you can balance the current using the taps in the tuner; hence the dual ammeters in series.

[W2VW]

On my old antenna I got up to 18 amps to the feeders with a 1500 watt test carrier on 160.

That’s one of the reasons I don't recommend placing the meters in series with each leg of the feeders. They should only be there to insure balance between both sides of the coupler, not to measure power. That should be done at the coupler's input side. The meters really need a suitable ground return to keep them from running away with excessively high current.

Ammeters in series with the balanced load will not be of much use to measure absolute power unless the impedance is known accurately. They do not respond to the reactive portion of the impedance either.

They are a great way to find out if certain designs are less lossy than others especially with short doublets. They are also useful for finding out if the coupler is in the unfortunate position of being right at a voltage node.

Looking at the schematics posted, I'd expect someone to be in for disappointment. The link coupled circus should be constructed with provisions for series and parallel connections if it is going to be used across all H.F.

I'd also lose the grounded centertap as mentioned before. It isn't going to fix an unbalanced condition. Doublets are pretty forgiving in this sense anyway. It will most likely not be the best way to feed the antenna as a vertical Tee either.  

[W2VW]

I did read your last post - I read every post in the thread, but I still don't know what your strategy is here.  Dave VW mentioned he's read 18 amps in series, and you responded that you don't like that idea for that reason. 

If you don't put them in series with what you're trying to measure (the balance of the antenna and feeders) I don't know how you can measure it.  The RF ammeters are essentially shunting all your energy to ground and are only measuring the ability of the transmitter to supply current. 

Are you trying, at some relatively low transmitter output, to measure the inherent balance of the tuner itself, or find the taps on the coil where it's balanced?  That's fine if the ant/feeders are 100% balanced but very seldom will they be, due to surrounding objects, etc.  That's why you can balance the current using the taps in the tuner; hence the dual ammeters in series.

John, he's trying to measure unbalanced current. Nice hat. 

[flintstone mop]

On my system I see 3 amps of antennna current running legal limit. M

Define "legal limit"

Contrary to popular belief, if it’s only 375 watts, then it’s too low. You can actually run more than that and still be perfectly legal.

There is still a real misconception among hams as to what is actually allowed.

YUP You're correct. I can run 1kw carrier and modulate so that PE.P. is 1500W. A neat trick to do.

Fred

The 18 amps is not some scientific thing. It tells me that the operator probably has a low dipole to the ground. I had a mess of a set-up in my early days on A.M. and a T-368 (300w)and a 25 foot high dipole indicated 10amps of antenna current. Today I have a VEE 70 feet in the air and 375 W carrier and 3 amps per leg.

[W2VW]

That 18 amps was bogus, and that was the point, it wasn't accurate. Each meter was probably reading current from both sides of the feedline combined plus coronal buildup and interference from surrounding objects, etc.


73’s

Noooooo. It was not bogus. It was a 124' doublet being used on 1.885 mhz. The point is very large current and voltages can be found when one uses short radiators.

[W2VW]

P.S. I don't need any series tuning here on my end. My center-fed Zepp has a fairly short feedline and parallel tuning works perfect on all HF bands. Already checked it.

73’s

I'd go play the lottery right away with luck like that.