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[Articles Home]  [Add Article]  

Build a QRP Tuner/Bridge

from Stephen Reynolds, W4CNG on February 4, 2008
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"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles."

QRP Long Wire Tuner/Meter

9-8-2001

By Stephen Reynolds W4CNG

Training Director North Fulton A.R.E.S. WWW.FULTONARES.ORG

Here is another quick and easy project with medium mechanical skills, and less electronic skills to Home Brew a QRP Tuner/Reflected power meter all in one small box. After having just swapped my FT-100 on a new FT-817, I am in the process of getting the rest of the goodies must be gathered to make the portable QRP station functional. I have the 20 Ft telescoping fishing pole, made a mount for it to hold one end of a long wire antenna. That takes care of the antenna, now how to tune it? I already had an MFJ 949E, but that is a large tuner with a not QRP forward meter. MFJ also makes a Random Wire Tuner model 16010. Fits in the palm of your hand nicely. Next you need an SWR Bridge to help you tune it. The 949E has a cross needle assembly which gives you what you need for fast and easy tune up. However in QRP, the general rules are the smaller the better. This project is about $80 for all new to re-package.

After doing some tinkering with an existing Radio Shack SWR bridge with the tuner, the idea arrived to meld the two together. How small can you get the box to hold what was necessary to have a workable tuner/meter. I first disassembled both the SWR bridge and the tuner. I the worked out what had to be on the front of the box, then what was needed on the rear of the box. The front has to have the Meter, inductor switch, and tuning capacitor. The rear needed the Radio input connector, and the antenna output connector, plus a ground connection and a banana plug. This combination works well for long wire antennas, as the random wire tuner should be at the antenna end of the cable not at the radio. Then how small can the box be and hold it all. It comes out to 5x3x2 and is just a little bigger than a business card as seen below in picture 1.

0x01 graphic

The next thing is laying out everything so it has it's own space. The box will be cramped when completed. The holes for the inductor and capacitor were drilled first (at a closer spacing than in the original box). You will notice that the inductor has a torqued to the left look. That was done to shorten the front to rear distance so it would fit in the box. Next came the meter hole (rectangular). I drew the outline of the meter top and sides on the inside of the front panel. Center drilled it and enlarged the hole to 1 dia. This gave plenty of room for the Nibbler to cut the hole. 15 minutes later it is done, and deburred. The meter was epoxied to the front panel with 5 minute epoxy. There was no other easy way to mount it, or space for brackets. That took care of the front. The finished front panel is shown below (rear view). You can also see the Radio connector on the rear panel along with the SWR Bridge board behind the tuner inductor. To reduce the parts count only the Reflected side of the bridge is used. I already have a FWD power meter on the FT-817. The meter only reads reflected power, so you tune it for a minimum reading.

0x01 graphic

Between the SWR meter and the tuner you have 4 chassis mount coax connectors and to use them requires a jumper cable. The electrical layout here places the SWR bridge first after the radio, feeding the tuner, with the tuner output at the antenna connector(s). You can see that in the picture above also.

I used the bridge measurement board as the input side of the rear panel, and removed the Antenna coax connector from that board. It is positioned where the Radio connector is on the right rear of the panel. Looking at the rear panel from the rear, you can see the red and black banana connectors for the ground and antenna connection. I also had to nibble a bit of the top of the box to make it fit over the antenna connector.

0x01 graphic

The Tuner input wire (white wire on bridge board) was soldered to the SWR bridge antenna connection. The tuner output was connected to the original SWR Bridge antenna connector re-used and repositioned in the upper right hand portion of the picture.

0x01 graphic

Both antenna ports are connected together (SO-239 and banana post. The ground pin of the banana post is grounded to the right hand shield of the bridge board. It is a little tight to get a picture of that.

To complete the assembly there are only 8 soldered connections to make. Plus/minus of meter, tuner input, tuner output (2), antenna output, banana plugs (2). That is a real simple project. How is the performance. I can load a 20 ft piece of wire on all bands 80-10 meters laying on the floor of my shop. To make a counterpoise, I used a 3 pin AC plug with a 4ft piece of wire connected to the Safety ground pin only, connected to the black banana terminal. Outdoors I would use the longest piece of wire I could string between one or more support devices.

CAUTIONARY NOTE: A word of caution here on using power safety grounds as counterpoises, you must verify that the outlet is properly wired using a commonly available $10 tester available at better hardware and home improvement stores. When out on field day or at other occasions where a long extension cord is used to supply power for operation, it is common to use the safety ground for a counterpoise. A long wire antenna will always work better with the longest wire and counterpoise you can rig and use.

Good luck and have fun.

Member Comments:
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Build a QRP Tuner/Bridge  
by LNXAUTHOR on February 4, 2008 Mail this to a friend!
- i love hardware projects like this... my favorite is the Norcal BLT and NoGaWaTT... building useful tools for the shack, bench, or field are just the 'ticket' IMHO for getting folks interested in ham radio...
 
Build a QRP Tuner/Bridge  
by N9ESH on February 4, 2008 Mail this to a friend!
A tuner, like the one in the article, is much too large for someone like me to lug around, not to mention the FT-817. When I take to the field, everything I carry must fit into my backpack. This includes a KX1 or DWS-II, small battery pack, paddles, headphones, note pad and “antenna system”.

The antenna I find that works best, in my experience, is an end fed half wave. Just toss it in a tree with a length of string and a weight. Only takes a minute or two to set up. A simple tuner made from a T50-2 toroid and polyvaricon capacitor matches the ~5000 Ohm antenna impedance to 50 Ohms. I built several from plans found on the Internet. One is in an Altoids tin and another in a salvaged Pomona box. They both have their own W7VE LED SWR indicators built in.

The tuner in this article is perfect for portable set-ups such as summer homes, park bench operating or emergency use where you won’t have to lug the stuff for miles on your back. It looks pretty beefy, too. Probably can handle much more than QRP power. My little “toy” tuners can only handle about 5 watts.

72/73
Jim/N9ESH
 
RE: Build a QRP Tuner/Bridge  
by AB9PZ on February 4, 2008 Mail this to a friend!
Nice little project. Just curious though.....do Field Dayers, etc really use the ground wire from the extension cord as a counterpoise for the antenna? I guess if it works then you can't argue, but I'm just wondering if this is something that's common. Seems like asking for trouble, if it's not grounded properly.

Cheers,
Brad
AB9PZ
 
RE: Build a QRP Tuner/Bridge  
by KJ4AGA on February 4, 2008 Mail this to a friend!
Keep stuff like this coming! I enjoy reading about and building almost anything electronic and I'm glad to see articles like this one.
 
RE: Build a QRP Tuner/Bridge  
by K6AER on February 4, 2008 Mail this to a friend!
“A tuner, like the one in the article, is much too large for someone like me to lug around, not to mention the FT-817. When I take to the field, everything I carry must fit into my backpack. This includes a KX1 or DWS-II, small battery pack, paddles, headphones, note pad and “antenna system”.

Nice article but I might add if the tuner shown in the article won’t fit in a back pack that is a very small back pack.
 
RE: Build a QRP Tuner/Bridge  
by K6AER on February 4, 2008 Mail this to a friend!
BTW is there a schematic for the above tuner?
 
RE: Build a QRP Tuner/Bridge  
by W9OY on February 5, 2008 Mail this to a friend!
Try

http://www.cebik.com/link/l-bal.html

look at the "single ended" schematic in fig 2

73 W9OY
 
RE: Build a QRP Tuner/Bridge  
by KC8VWM on February 5, 2008 Mail this to a friend!
To complete the assembly there are only 8 soldered connections to make. Plus/minus of meter, tuner input, tuner output (2), antenna output, banana plugs (2).

---------

I suppose if I wanted a smaller, more miniature version of this project for my FT 817 (Altoids tin QRP version) I would probably start by loosing the big meter in favor of using 5 or 6 L.E.D.'s. When working QRP, I am basically looking for a ballpark SWR figure because most antenna installations are usually temporary in nature.

I wouldn't likely use an etched PC board inside this design because there are so few components ( a few capacitors/resistors etc.) to bother with going through this extra effort anyways.

I would probably change the SO-239's, replacing them with BNC connectors and do away with the banana plug antenna jacks in favor gaining a further reduction of any practical space and overall size. Using smaller dial knobs would probably finish the project.

I enjoyed the article. I understand why it wasn't necessary to publish a schematic for this simple design. It's probably because the article was originally intended to provide lot's of potential and practical thought for creating your own simple and practical design.

Now where did I misplace my soldering iron this time?

73 de Charles - KC8VWM
 
Build a QRP Tuner/Bridge  
by 4S7CP on February 6, 2008 Mail this to a friend!
Dear OM!

It is very nice and simple, easy to build and very useful.

Thank you very much.

We r expecting more from you.

Colin 4s7cp
 
Build a QRP Tuner/Bridge  
by W3HR on February 6, 2008 Mail this to a friend!
Probably works just as well as, or better, than any MFJ tuner! ;)
 
RE: Build a QRP Tuner/Bridge  
by N9ESH on February 7, 2008 Mail this to a friend!
“A tuner, like the one in the article, is much too large for someone like me to lug around, not to mention the FT-817. When I take to the field, everything I carry must fit into my backpack. This includes a KX1 or DWS-II, small battery pack, paddles, headphones, note pad and “antenna system”.

”Nice article but I might add if the tuner shown in the article won’t fit in a back pack that is a very small back pack.”

Actually, the backpack I use is fairly large. However, I do have to carry a lot of other necessary items besides radio stuff. One thing I don’t need is coax or twinlead, ground leads, masts or a tuner as large as one of my rigs. You wouldn’t believe the time and effort spent by a hiker to thin out the items in a backpack.

Jim/N9ESH
 
Build a QRP Tuner/Bridge  
by KG4IVT on February 8, 2008 Mail this to a friend!
I'm kind of new to this stuff, but I had a question. Would it be easy/practical/etc to bundle together some resistors on a board and put it in a small tuner like this for a low power dummy load? If so, could a small toggle switch be used to switch the antenna output to the load or would you need a different switch?
 
RE: Build a QRP Tuner/Bridge  
by KL7AJ on February 9, 2008 Mail this to a friend!
ESH:

I think this little feller is about as compact as you can make it...and with the ferrite tank coil, it's even smaller for the same power capacity than it would be otherwise. How would one shrink this any more?

Eric
 
RE: Build a QRP Tuner/Bridge  
by KL7AJ on February 9, 2008 Mail this to a friend!
Actually, a flashlight lamp would make a nice dummy load too. For H.F. frequencies, it's good enough for general tuner-upper purposes.

Also...a neon lamp loosely coupled to the output terminal is a good indication of maximum output voltage...and will actually work with QRP power levels on a high impedance antenna. (Won't work with a Short random-wire though)

eric
 
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