Johnson Matchbox Question

[n3lrx]

I have a Johnson Matchbox Model 250-23 just wondering how under rated it is it's suposed to be 375W but is that carrier rated or PEP rated?

I'm putting together a Class E transmitter and planning on about 200W carrier will this tuner handle a PEP of around 500 or so watts  or should I invest in a larger one.. I'm hoping so because the only thing within budget right now is an MFJ (Mighty Fine Junk) so hopefully old faithful will handle the full output.

If not is there any mods I can do to it so it can handle the PEP?

The only mods that have been made to it is built-in T/R and a simple diode detector for off air listening. Other than that it appears to be factory original.

The antenna I plan to use is a full length 1/2 wave dipole fed with 450 ohm ladder. (The most common brown plasti-coated stuff, I've debates as to whether it is 450 or not.) Hopefully the apex will be around 60-70' not quite flat top but I'll get it as close as I can.

TNX!

[Pete, WA2CWA]

If this is the small one, I believe the Max. rating is 275 watts. Ran one for years, but never attempted to run more than 125 watts through it.

[K1JJ]

The 275W rating on the Junior is supposedly an AM carrier rating.  Though, last year at HossTraders or was it Boxborough? a 375W watt class E rig was arcing the hell outa one.

A 500W pep class E rig is about 100W carrier or so, so it shud be fine. Others can comment, but from the ones I've owned, a lot depends on the impedance of the feeders where they connect to the tuner - but it shud handle a 200W carrier in most circumstances.

Build up a simple, cheap  JJ Super Tuner rated at 5KW and never worry about it again!

http://amfone.net/Amforum/index.php?topic=6404.msg46882#msg46882

Do more searchs here - there's more info..

73,
T

[KF1Z]

Even if that's a max rating of 275 watts at carrier, you likely won't be happy with it for class-e.
For the reason K1JJ mentioned....

Most of these class-e rigs are capable of some awesome modulation peaks...
Arcing over is a definate concern......


Is the e-rig your own design?
Or a "QIX" ?

doesn't really matter in relation to your post, but curious......

I built one of the 'newer' designs, well, same thing he had at Hosstraders....
My normal carrier level is at 280 watts out, at 320watts DC  IN. (and that efficiency is a tad low).


At any rate.....
This is prolly one of those situations where bigger is better!

As K1JJ said, build a biggun' and not worry about it!


Besides, If you 'upgrade' you class-e rig later, you'll need the extra "zorch-control"....


Bruce g

[Steve - WB3HUZ]

Yo Randy. We're are you living these days? Hope to hear YellRX Radio on the air soon. It's been a long time.

Not sure about all the Class E hype. PEP is PEP. The tuner doesn't know if it's coming from Class A,B,C,D,E,F,G or Q. If the box is rated for 275 watts of AM carrier, that's 1100 watts PEP at 100 % modulation. So, if you are only running 500 watts PEP, flash overs will be as a result of some weird impedance in the antenna system.

[K1JJ]

The HuzMan sez: The tuner doesn't know if it's coming from Class A,B,C,D,E,F,G or Q.

Don't be talking about no class Dog rigs, caw mawn.

[KF1Z]

My point about it being class-e, was that generally, a class-e transmitter is capable, and in most cases run at up to 150% positive modulation .... where a lot of other transmitters are operated at 100% (or below).

That's the reason I mentioned it......

As you said pep is pep.................

[W2JBL]

 the little Matchbox works to 1200-1300 PEP and 300 watt carrier OKfine for me. this also includes full strap SSB and CW. a few years ago Timtron was up at my Adirondack station, and sat down to tune up my Clipperton L leenyar into it. i winced as he loaded it up to full strap, and commented i did not think it was a good idea. Tron said no problem- they handle it. and this one has on all bands for two or three years. your results may vary. in this case the antenna is a 166 foot centerfed with ladderline at about 70 feet. i have not tried this feeding coax, or a single (unbalanced) wire antenna. i use it at that station because it's what i had at the time. my preference is still for the KW Matchbox or (best tuner around) something homebrew/link coupled like the "JJ" tuner.

[n3lrx]

Coolness.. I'll see what I can come up with. I've been off the air for ages and I'm looking to get back on ASAP. If my little Viking can't handle it I'll build a JJ tuner..

Anyway, I'm building a K7DYY Jr. I'd like to build one of steves designs but I'll have to accumulate some more parts for that.. I'll tackle one later but for now something simple is what I need to get on the air with.

Hopefully this time with a little more outpoot potential and more altitude on the antenna I won't be so peeeeeeee issssss weak!

TNX OM's!

[n3lrx]

Huz Man.. I'm in Old Bowie right now.. Not sure if you heard or not but Ed KC3SV passed last December. His family asked Rob (N3RLL) and I to clean up the house for them since they live out of state. In the process of doing so I met the landlord and he asked if I wanted the place.. So I took it.

Tis but a small shack but there's allot of land and alot of tall trees.. There was a 75m diaper pole ready to go but a falling branch took one of the legs down. I'll probably replace the whole thing it's been there for as long as I can remember so god only knows how much it has streched so I might as well build a new one so I know what length the wire is.

I'd thought about building a Bazooka, I've got a full roll of 75ohm cable TV twin lead. (Two coax runs fused like zip cord.) Might as well put it to some use. I've been waiting for fall and the foliage to die down and the leaves to drop off the trees so I can launch some wires. I've tried before but to no avail. The briars are so thick in the woods behind the house it's nearly impossible to get through without getting ripped to shreds. Not to mention handling the wire without getting it into a tangled knot in the briars.

[WA1GFZ]

both flash boxes are sensitive to feeder length. Add the length of the feeder to one leg of the antenna. That length wants to be away from a 1/2 wave multiples. As Huz said usually a problem with Z rather than power.  The balanced / balanced tuner has more range than a flash box and produces the same feeder current. The homebrew link tuner properly built will also work fine. It all depends on your junk box loot.

[W2VW]

We had a bum dipole/feeder length at the last K3L. The kilowatt flashbox hated it. Double balanced tuner worked FB but took a lot of time to setup.

[W1IA]

I have run 350 watts carrier at 2200 watts peak through mine and has never flashed over...mind you the antenna was cut to resonance and fed with ladder line.  Reactance numbers far out of the norm could be a problem?

Brent

I have a Johnson Matchbox Model 250-23 just wondering how under rated it is it's suposed to be 375W but is that carrier rated or PEP rated?

I'm putting together a Class E transmitter and planning on about 200W carrier will this tuner handle a PEP of around 500 or so watts  or should I invest in a larger one.. I'm hoping so because the only thing within budget right now is an MFJ (Mighty Fine Junk) so hopefully old faithful will handle the full output.

If not is there any mods I can do to it so it can handle the PEP?

The only mods that have been made to it is built-in T/R and a simple diode detector for off air listening. Other than that it appears to be factory original.

The antenna I plan to use is a full length 1/2 wave dipole fed with 450 ohm ladder. (The most common brown plasti-coated stuff, I've debates as to whether it is 450 or not.) Hopefully the apex will be around 60-70' not quite flat top but I'll get it as close as I can.

TNX!

[K1JJ]

I have run 350 watts carrier at 2200 watts peak through mine and has never flashed over...mind you the antenna was cut to resonance and fed with ladder line.  Reactance numbers far out of the norm could be a problem?

Yep, I believe it all comes down to impedance and the spacing of the capacitor plates. The 275W rating is probably rated for say, a 600 ohm line. At 2000 ohms maybe it will handle only 100W before flashing over. At 100 ohms (if you could match it)  maybe that  Matchbox would handle 600W carrier without flashing over... though the plexiglass coil might not like it.

In a pinch, by changing the feedline length, most arcing problems can be cured. Still, for multi-band use, there will always randomly be a freq where it is high impedance for arcing problems. The solution is to build up a big tuner with a vacuum variable to be bulletproof - and fergitaboutit.

T

[Ed/KB1HYS]

I've started one of those BFT's. (Big Freakin' Tuner)  I've got about 40 turns of 3/8ths copper tubing on a 4.5 inch PVC form.  (50ft roll from HD)  I picked up a 250pf Johnson cap that looks like part of a KW tuner.  I'd like to tune 160, 80, and 40 into a folded 160 dipole,  ya think Is 250pf enough??  I can parallel some Door knobs across it if need be...

[K1JJ]

I've started one of those BFT's. (Big Freakin' Tuner)  I've got about 40 turns of 3/8ths copper tubing on a 4.5 inch PVC form.  (50ft roll from HD)  I picked up a 250pf Johnson cap that looks like part of a KW tuner.  I'd like to tune 160, 80, and 40 into a folded 160 dipole,  ya think Is 250pf enough??  I can parallel some Door knobs across it if need be...

Ed,

Sounds like a good start so far.

Getting away with a 250 pf capacitor depends on what particular antenna feed impedance you end up with on 160M. You see, with the JJ Super Tuner design, there are many combinations to get a 1:1 match. I recommend to have clip leads for both the antenna feeders AND the capacitor connections to the coil. Various combos of this will produce widely varying capacitor settings.

So the most you will lose is some additional versatility to find all possible combos to match if you run out of range. 250 pf is probably just enough to squeak by on 160M for most cases I think. . I usually use a 500pf vac cap and it is never all in. The door knobs may work or may blow up depending on the power and ratio of current you put through them compared to the air variable.

Let us know how you make out with 250 pf with your particular feedline length on 160M, OM. And please post a picture when completed. Pics always encourage more guys to build them up - they soon see how easy it is to never worry again about arcing or matching ANY line on any band.

Details:   http://amfone.net/Amforum/index.php?topic=6404.msg46882#msg46882

http://amfone.net/Amforum/index.php?topic=5931.0

73,
T

[Steve - WB3HUZ]

The coil and the capacitor must resonate on the lowest freq you plan to use the tuner. This does not count needing to 'tune' out any reactance. Do the math and plan accordingly.

[K1JJ]

The coil and the capacitor must resonate on the lowest freq you plan to use the tuner. This does not count needing to 'tune' out any reactance. Do the math and plan accordingly.

Yes, normally this is correct ....

On this particular quad clip lead design we get more versatility which may require bigger caps than other designs. You will find that by tapping the capacitor in on the coil, (higher Q)  it will sometimes match feedlines that it normally would not. ie, More C and less L (still resonant) can sometimes match combinations that are not easily done when keeping the l/C ratio fixed.

For example, sometimes the capacitor needs to be all the way in and the coil less while tapping the feeders out on the end of the coil ,etc.   Being able to hit all combinations like this is what gives the tuner it's wide ability to match anything. (except very low impedances) Whereas tuners like the Matchbox are stuck in a certain L/C ratio unless you go inside and start tapping.

That's why premarked taps with copper tabs soldered to the coil for each band are a joy. You can QSY in no time flat.

T

[Steve - WB3HUZ]

I'm pretty sure tapping in on the coil with the cap is no different than (or about the same as) shorting out turns, that is those turns outside the cap taps aren't really involved. So the turns that are inside the cap tap must still produce a resonant circuit. No resonance, no power transfer, no tune, no signal.

[K1JJ]

I think where the feeders are positioned has a great affect on Q of the coil, thus a way to compensate for this is by changing the ratio of L/C.  It seems in marginal matching situations, changing the L/C ratio makes the difference to get 1:1 with a given feeder tap.

At least that's the way it "appeared" in actual practice the last time I used a JJ super tuner.    (It was over Craig's and I was pretty stoned, caw mawn) 

T

[Steve - WB3HUZ]

True. Generally, the more you tap down on the coil with the feeders, the higher the Q. At some point it's better to go with a series fed connection rather than tapping down further to achieve a match.

[W2VW]

Double balanced tuner can also get away with less copper a lot of times. Beating a dead horse is one of my hobbies.

[Steve - WB3HUZ]

If you consider using two big roller inductors as less copper.

[W2VW]

If you consider using two big roller inductors as less copper.

I've yet to use much of the L on the 2 inductors under any situation.

[Steve - WB3HUZ]

Look at all that copper you are wasting. Terrible!