Antenna transmatch

[KC2UGV]

Ok, so I'm browsing for an antenna transmatch now. However, at near $100, my wife would shoot me if I bought one, since i just spend $375 on a radio+power supply (But, I did get a sweet D104 mic).

So, the question is: Until I buy a transmatch, how do I go about tuning my antenna, short of antenna for each band (Not all that adverse to it however, if it's the case).

I only ask, because I have no analyzer right now, so I can't really analyze my antenna system (It's a dipole, sorta. In the attic, like this:



^ <-- Feedpoint
/ \
| |
| |
| |


It's "cut" for 20M, since I figure that would work as a eight wave on 80, quarter wave on 40, and a full wave on 10.

I'm only running very low power into the antenna currently, since I have no clue what the SWR or Impedances for each of the bands really are. I figure I wont cook my radio on low power.

So, how can I do a down-and-dirty tuner? Or some other trick that I don't know about? Or, would multiple antennas be the best bet? Am I being overly concerned about it?

[w3bny]

Honestly,

Ask to borrow one till you can afford to find/buy/build one. Unless your running a full bucket, you can find pretty good <200w tuners for pretty cheap.

As for plans, ARRL has some great home built plans

[X-Rated]

Wow. you are into some heavy duty stuff. Like when you want to use the 20M antenna for 80M, you will need to tune that and you will need to know a lot of stuff and specifically. Small changes like sneezing and differences in leg offsets will be pretty tremendous.

When you cut a regular dipole, the Q of that circuit is pretty low. Therefore it will tune over a relatively large range. When you cut an antenna short, you can make up for it with loading components and tuning stubs to offset the stray reactances. But the tuning range suffers because the Q of the antenna is high when properly tuned. Hitting it by accident will be difficult at best.

If you just want to drive it and find out what you can do, then go for it and run 5W. Your 840 should work pretty safely mismatched at 5W.

I have an MFJ 969. A lot of people badmouth MFJ tuners and rightly so. But this one is pretty good. You can sometimes find them for a little over $100. You really need a tuner.

[KG4CGC]

I have not built one yet but they look to be pretty easy to build. Use heavy duty parts all around. No need for fancy roller inductors.

[X-Rated]

I have not built one yet but they look to be pretty easy to build. Use heavy duty parts all around. No need for fancy roller inductors.
I built a tuner when I was a kid and it worked pretty good. I did bump it a few times and suffered RF burns. That was so much fun. I need to do that again.

[KG4CGC]

I have not built one yet but they look to be pretty easy to build. Use heavy duty parts all around. No need for fancy roller inductors.
I built a tuner when I was a kid and it worked pretty good. I did bump it a few times and suffered RF burns. That was so much fun. I need to do that again.
I'm sure that bumping furthered your education. Yes, as adults, we understand the importance of electrical safety in our everyday lives.
I'm just thankful that you are still around to express the importance of possibly shielding the unit. :angel

[KC2UGV]

I have not built one yet but they look to be pretty easy to build. Use heavy duty parts all around. No need for fancy roller inductors.

Well, I was thinking the building wouldn't be too hard. I've seen some designs online to look at. Are the variable air caps really close tolerance?

For example, if I build a butterfly and don't test the capacitance and do a "best guess", it should still work as long as I tune for "least noise", or no?

[KG4CGC]

I have not built one yet but they look to be pretty easy to build. Use heavy duty parts all around. No need for fancy roller inductors.

Well, I was thinking the building wouldn't be too hard. I've seen some designs online to look at. Are the variable air caps really close tolerance?

For example, if I build a butterfly and don't test the capacitance and do a "best guess", it should still work as long as I tune for "least noise", or no?
Use it with a VESWAR meter at low power and before keying tune for most noise.
SWR meter and most noise.

[N8YX]

What size attic?

If large enough, consider running a loop all the way around it, then build separate - switchable - matching networks for each band you wish to cover. Remotely activate each when you wish to change bands,

[w3bny]

Or just run Echolink and Hamsphere... :geek: :doh:

[KC2UGV]

What size attic?

If large enough, consider running a loop all the way around it, then build separate - switchable - matching networks for each band you wish to cover. Remotely activate each when you wish to change bands,

If I'd hazard a guess, I'd say 30x60ft...

[N8YX]

What size attic?

If large enough, consider running a loop all the way around it, then build separate - switchable - matching networks for each band you wish to cover. Remotely activate each when you wish to change bands,

If I'd hazard a guess, I'd say 30x60ft...
That would be great for such an antenna.

30ft x2 + 60ft x2 = 240ft; you can always switch in extra inductance for 75/80 if necessary. All other bands would be fairly easy to match.

[KC2UGV]

What size attic?

If large enough, consider running a loop all the way around it, then build separate - switchable - matching networks for each band you wish to cover. Remotely activate each when you wish to change bands,

If I'd hazard a guess, I'd say 30x60ft...
That would be great for such an antenna.

30ft x2 + 60ft x2 = 240ft; you can always switch in extra inductance for 75/80 if necessary. All other bands would be fairly easy to match.

Don't loops need those monster butterfly caps at the feedpoint? I'm not averse to that at all, but the one's I saw had these huge 5000V variable caps at the feed point.

[KC2UGV]

And I need to use ladder line for a loop right, or else it defeats the purpose of the loop?

[N8YX]

Don't loops need those monster butterfly caps at the feedpoint? I'm not averse to that at all, but the one's I saw had these huge 5000V variable caps at the feed point.
What you can do is to use some doorknobs (7.5kV) in parallel with smaller, "trimmer" style variables with large plate spacing. Switch in and out per band (as needed) via relay.


And I need to use ladder line for a loop right, or else it defeats the purpose of the loop?
Nope - you can use coax feed up to the matching network, which must transform an unbalanced low-impedance input to a higher impedance balanced output.

You can even build the balun out of coaxial cable if you wish. This way you'll not get into core-saturation issues.

[N8YX]

Food for thought - check the following:

http://www.eham.net/forums/Elmers/227332

I have a couple more references that I'll post later.

[KC2UGV]

Food for thought - check the following:

http://www.eham.net/forums/Elmers/227332

I have a couple more references that I'll post later.

Sweet! I think this is getting me on the right start. one of the posters (N3OX) talks about how to build the matching networks and the inductors :)

Thanks a bunch! And to think I tried to steer clear of eham...

[ka4dpo]

You can build an L network tuner for next to nothing. All you need is a coil of about 25 turns spaced one wire width and about 2 or three inches in diameter, and one 10 to 200 Uf variable capacitor. You can find these at a hamfest or on Ebay for very little money. The tuner can be built on a pice of wood and will load darn near anything. If you are an ARRL member here are two links that will help you build a dirt cheap all bander and tuner that really works.

http://www.arrl.org/members-only/qqnsea ... %5B%5D=QST (http://www.arrl.org/members-only/qqnsearch.html?search=1&words=window+sill+antenna&name=&call=&year=&month=&selpub%5B%5D=QST)

Look at the 1957 article by Lew McCoy. It shows how to build the tuner. You can substitute any length of wire for the whip, just make sure you have a counterpoise or, you can connect the coax from your dipole to the tuner and use it that way. Just some food for thought, the most expensive tuner Ten Tec makes is an L network. These have the lowest internal loss of any matching network and you can even connect a Balun to it and run ladder line or TV twin lead if your poor (that works fine by the way, I've done it). So, hope I helped you.

John.. :cheers:

[KC2UGV]

Ok, so I think I might venture into building a tuner. A T-network one (Seems easiest to build), and since my theory is a little weak, I need as few things that can go wrong as possible.

For building the air capacitors, what should I aim for on plate spacing? 1/16"? Is that too much, too little?

I saw a plan that uses metal venetian blinds as the stock material (I got lots of those :) , but they are laminated (Painted). Would this work as stock? I'd think so, but wondering if bare metal vs. painted metal is a big deal.

And, how many plates should I look at? 10? 20? 100?

[N8YX]

Once again...eHam to the rescue:

http://www.eham.net/articles/5217

A bevy of capacitor calculators:

http://www.csgnetwork.com/capacitorinfoconverters.html

In this application - and for 100w RF power input - I would use a plate spacing which yields at least 1kV breakdown in humid air. 2.5kV would be better.

The dielectric strength of air is as follows: 3 × 10e6 V/m

Using that figure, one may extrapolate the distance required between plates for a given breakdown voltage.

[KC2UGV]

BOOKMARKED! :mrgreen:

[KG4CGC]

I've been interested in building variable capacitors but I also see this as a bit esoteric for me at this time. I will continue to read up on it though.
Need Vcaps? Hamfest, hamfest, hamfest usually 5 bucks. I have also bought them in package deals on ebay.

Now if you're really good, consider sheet metal and plywood. Do your mock up on cardboard first. Yeah, I could easily spend on tools but once you got'em ...

[N8YX]

Need Vcaps? Hamfest, hamfest, hamfest usually 5 bucks. I have also bought them in package deals on ebay.
Good luck finding the really large - useful at QRO - ones.

I have in my possession a 200pF/section, 50kV spacing butterfly variable. Big Rat Bastich.

No, it isn't for sale.

Plans are to build a remote tuner for a sloper array that I hope to construct some time in the next few years.

[KG4CGC]

Need Vcaps? Hamfest, hamfest, hamfest usually 5 bucks. I have also bought them in package deals on ebay.
Good luck finding the really large - useful at QRO - ones.

I have in my possession a 200pF/section, 50kV spacing butterfly variable. Big Rat Bastich.

No, it isn't for sale.

Plans are to build a remote tuner for a sloper array that I hope to construct some time in the next few years.
Thank you sir and good luck to you as well LOL!
They're there but it's like fishing.

[N8YX]

They're there but it's like fishing.
I got hold of a couple at Dayton a few years ago. Where I really made out was on some big, honkin' 6 and 8-position, 5-deck ceramic band switches. 5kW+ units in anyone's book.

$1.00/ea.

'DSG was mumbling "WTF?" up until I showed her how much the things were fetching online.

[KC2UGV]

Need Vcaps? Hamfest, hamfest, hamfest usually 5 bucks. I have also bought them in package deals on ebay.
Good luck finding the really large - useful at QRO - ones.

I have in my possession a 200pF/section, 50kV spacing butterfly variable. Big Rat Bastich.

No, it isn't for sale.

Plans are to build a remote tuner for a sloper array that I hope to construct some time in the next few years.

Ok, dumb question then. At 100W out, do I need butterflies, or can I get away with those little trimmer ones you find in radio's?

[N8YX]

Ok, dumb question then. At 100W out, do I need butterflies, or can I get away with those little trimmer ones you find in radio's?
Use the formula shown above, measure the plate spacing and determine the breakdown voltage. If the trimmer units are even close to 1kV spacing, they'll work. (IIRC, the output loading caps in a couple of my amps use 750v spacing @ 600w PO - but they will arc over if the amps are loaded incorrectly.)

Most of the trimmers used in BC receivers are of the 100v (or less) variety and use Mylar as a dielectric. Good for QRP; certainly not good for 100w network input power if attempting to match a high-impedance load.

[KC2UGV]

Ok, dumb question then. At 100W out, do I need butterflies, or can I get away with those little trimmer ones you find in radio's?
Use the formula shown above, measure the plate spacing and determine the breakdown voltage. If the trimmer units are even close to 1kV spacing, they'll work. (IIRC, the output loading caps in a couple of my amps use 750v spacing @ 600w PO - but they will arc over if the amps are loaded incorrectly.)

Most of the trimmers used in BC receivers are of the 100v (or less) variety and use Mylar as a dielectric. Good for QRP; certainly not good for 100w network input power if attempting to match a high-impedance load.

Ah, got it. On with the butterfly then :)

[KC2UGV]

Ok, so I still plan on building a transmatch (Something about the making it, just does it for me), but for now, I found one I couldn't pass up:

Antenna Transmatch = $5

It's made by Ascom, looks to be about 25-30 yrs old, and came from an SK estate. All it says,"AS (ascom) Antenna Transmatch".

Have no idea yet what the specs are, but I'll figure it out from estimates, or I'll scrounge up a full circuit tester (Cap meter, inductance meter, etc).

I opened it up, and man, short of the plastic side panels, this think looks like it was built like a tank. Two butterfly caps (Have to be small rated, gap of about 1/16" or 1/32"), and it's a T network, I think.

And, I got my first Heathkit. A dummy load :lol: $5

[ka4dpo]

I guess I'm curios why you latched on the T network. An L network is easier to build, has practically the same impedance matching range and one other key operational advantage. A T network can, under certain load conditions become a high pass filter and will provide a false SWR indication. That is, the SWR meter will tell you that your system is tuned when it is not. Finding the right LC comination on a T network can be challenging because of this. A L network will only provide a match at the correct setting. Like I already said, some of the best transmatches ever made like Drake and Ten Tec use the L network. People have a mistaken notion that because something is simple it can't work well when in this case it is just the opposit. To provide a conjugate match you Xl (inductor) and Xc (capacitor), the L network gives you just that. Try it an see if I'm right......

And one more thing, for a 100 watt rig a capacitor plate spacing of 1/16 inch is more than adequate.

[KC2UGV]

I can make easily a variable cap, not so much a roller inductor. And all the designs for L's I've seen require a roller inductor.

Only reason.

The only reason I bought the match I did: It cost $5 :)

[N4VGB]

Don't worry Corey, RF burns are non-bleeding wounds. :rofl:

[KC2UGV]

Don't worry Corey, RF burns are non-bleeding wounds. :rofl:

lol, that's what I've read too :) Leaves nasty scars though.

[N4VGB]

Don't worry Corey, RF burns are non-bleeding wounds. :rofl:

lol, that's what I've read too :) Leaves nasty scars though.

Gives new meaning to the term "black hole", since that's exactly what RF usually leaves on ya, a nice "black hole". Sometimes a big and deep "black hole", all the way to the bone! :shock: :rofl:

You ain't a real ham until ya can show some scars! :rofl:

[ka4dpo]

I can make easily a variable cap, not so much a roller inductor. And all the designs for L's I've seen require a roller inductor.

Only reason.

The only reason I bought the match I did: It cost $5 :)


No they don't. You can use taps and a wafer switch or, you can use a wire with an aligator clip to tap the coil. Roller inductors are nice to have but totally unecessary for it to work. Just find the right tap locations for each band, solder a wire to that point on the coil and connect it to the switch location of your choice. Commercial tuners have done this for years and still do.

[KC2UGV]

I can make easily a variable cap, not so much a roller inductor. And all the designs for L's I've seen require a roller inductor.

Only reason.

The only reason I bought the match I did: It cost $5 :)


No they don't. You can use taps and a wafer switch or, you can use a wire with an aligator clip to tap the coil. Roller inductors are nice to have but totally unecessary for it to work. Just find the right tap locations for each band, solder a wire to that point on the coil and connect it to the switch location of your choice. Commercial tuners have done this for years and still do.

Hm... I'll give that a whirl then. Easier to wind a coil than to build an air cap...

[KG4CGC]

I can make easily a variable cap, not so much a roller inductor. And all the designs for L's I've seen require a roller inductor.

Only reason.

The only reason I bought the match I did: It cost $5 :)


No they don't. You can use taps and a wafer switch or, you can use a wire with an aligator clip to tap the coil. Roller inductors are nice to have but totally unecessary for it to work. Just find the right tap locations for each band, solder a wire to that point on the coil and connect it to the switch location of your choice. Commercial tuners have done this for years and still do.

Hm... I'll give that a whirl then. Easier to wind a coil than to build an air cap...
Another idea to building variable capacitors is to use double OR single sided PC board material.
A simple cap consists of two plates where the spacing can be regulated through a screwdriver device. The size of the plates determines max overall capacitance. This is also controllable through a remote device and will give you more room to fine tune in much the same way a rotary works with a reduction drive.

[N4VGB]

Hm... I'll give that a whirl then. Easier to wind a coil than to build an air cap...



You'll still need a variable cap. Single L section or balanced-balanced double L section, still uses a variable cap.

[N8YX]

Hm... I'll give that a whirl then. Easier to wind a coil than to build an air cap...



You'll still need a variable cap. Single L section or balanced-balanced double L section, still uses a variable cap.
Or a bunch of trimmers that can be switched in and out on a per-band basis.

I've got a neat little prototype 'Cubic' random-wire antenna tuner - remote controlled - which is set up exactly this way. The radio's band info output connector causes a rotary shorting switch inside the tuner to select an operating range...then the associated trimmer is adjusted for lowest observed SWR. This concept worked well for a narrow range of channels in each operational "band" but probably wouldn't lend itself to wideband operation in the amateur radio service.

Still, I plan on finishing the thing up some time then building an interface cable so I can use one of my Astro-D radios with it.

[KC2UGV]

Hm... I'll give that a whirl then. Easier to wind a coil than to build an air cap...



You'll still need a variable cap. Single L section or balanced-balanced double L section, still uses a variable cap.

Yeah, but I'd only have to make one, instead of two :)

[N4VGB]

Hm... I'll give that a whirl then. Easier to wind a coil than to build an air cap...



You'll still need a variable cap. Single L section or balanced-balanced double L section, still uses a variable cap.

Yeah, but I'd only have to make one, instead of two :)

Take your time Corey, the wife & kids are gonna kill ya anyway for filling the house with RF with that attic antenna. :rofl:

[KC2UGV]

Hm... I'll give that a whirl then. Easier to wind a coil than to build an air cap...



You'll still need a variable cap. Single L section or balanced-balanced double L section, still uses a variable cap.

Yeah, but I'd only have to make one, instead of two :)

Take your time Corey, the wife & kids are gonna kill ya anyway for filling the house with RF with that attic antenna. :rofl:

Nah, ended up putting it outside already in the tree. Now, she wants to kill me for having wires in the tree outside :)

[N4VGB]

Nah, ended up putting it outside already in the tree. Now, she wants to kill me for having wires in the tree outside :)



Ahhh, now you're in for the second lesson of ham radio. First time a huge flash and kaboom of lightning happens, it's you and that damnable ham radio antenna "drawing" that lightning! :rofl: