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Build a QRP Tuner/Bridge

Stephen T. Reynolds (W4CNG) on September 28, 2001
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QRP Long Wire Tuner/Meter


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 N3GWZ on September 28, 2001 Mail this to a friend!

What an interesting article! You did a great job with reusing existing equipment and repackaging into a small box.

I also use a long wire off the back of a Versa Tuner MFJ-901B. I use the FT-817 internal SWR meter for tuning. The long wire does a great job and is extremely portable. Winding the wire back on the spool is the hardest thing about using it tho :-)

Safety ground as counterpoise?  
by K3AN on September 28, 2001 Mail this to a friend!
The counterpoise in this case is the 4' of wire to the outlet, plus the 30' or more of Romex wire on the branch running back to the breaker panel and its ground rod. The counterpoise also consists of all of the devices (lamps, radios, TVs, etc.) plugged into the other outlets on that branch.

The capacitance between the green ground wire and the hot and neutral wires is probably 100 pF or more, so the signal on the ground wire is coupled to the black and white wires in the Romex. With the antenna consisting of just 20' of wire on the shop floor, I suspect that the counterpoise is probably doing most of the useful radiating!

Not a big deal if you run QRP, even if the kid is watching TV while sitting next to a metal floor lamp plugged into that branch. But in addition to radiating signals, that counterpoise also picks up signals, including all the noise from TV power supplies, dimmer switches, etc. and delivers them straight to the input of the receiver.

For this reason, I think I'll stick to using a separate counterpoise. What do the rest of you say?
Build a QRP Tuner/Bridge  
by W4CNG on September 28, 2001 Mail this to a friend!
An added note for a couple of E-Mails I received. The meter is connected to the reverse side of the SWR bridge. The FWD/REV switch was removed, so you use the two connections on the board where the REV side of the switch was previously connected to.

Steve W4CNG
Build a QRP Tuner/Bridge  
by HFHAM on September 28, 2001 Mail this to a friend!
Fine Business Old Chap !

At last, an article on Amateur Radio.

Many thanks to real hams like yourself who keep the flame alight !
Build a QRP Tuner/Bridge  
by HFHAM on September 28, 2001 Mail this to a friend!
P.S. I'm with K3AN on the counterpoise. I'd avoid using a power socket ground; not worth the risk (or the potential for RFI/TVI).
RE: Safety ground as counterpoise?  
by W4CNG on September 29, 2001 Mail this to a friend!
Please note that on the test I laid a wire on the shop floor to test, that was fine. In reality random wire antennas are not laid on floors, and counterpoises do not feed thru buildings, apartments for feet on end. In fact most desk lamps and other non grounded polarized outlets do not touch the safety ground that is grounded to the building frame several times more thru conduits and other non RF sources. The reference to Field Day long power feeds is true, a long extension cord with a safety ground back to the source of power (Generator) makes one whale of a good counterpoise at any frequency up to 160 meters if the cable is 100ft or more. Most noise (man made) is generated locally on HF, sometimes you have it, sometimes you do not. All antenna systems must have a good ground to reflect against, if not then you have a compromise.

73 Steve W4CNG
RE: Safety ground as counterpoise?  
by AB8JC on October 3, 2001 Mail this to a friend!
"All antenna systems must have a good counterpoise...." Nope, not true. Want examples? J-pole. Dipole or other forms of doublet. Quad. Loop. Enough?

Also, why spend $80 on this when you can pick up a BLT kit from NorCal QRP ( for $25? Yeah, a second BNC connector and another switch are needed to feed "unbalanced" antennas, so there's another $5. $30 max versus $80 for this thing? Don't get me wrong, it's nice to see a construction article, but why reinvent the wheel, at over twice the price?
RE: Safety ground as counterpoise?  
by W4CNG on October 5, 2001 Mail this to a friend!
Take half a dipole or doublet away and you have a long wire without a counterpoise, half of the dipole is the radiator, half is the counterpoise, or did I miss something in Antenna design 101 in 1965? I already had both items, so for an $8 box and a couple of hours labor built the box which is greater than the sum of both and far more convienent. I would guess there are lots of folks that have lots of things around that when "Married Together" can be greater than the sum of both as a stand alone item.
RE: Safety ground as counterpoise?  
by KC7YRN on November 5, 2001 Mail this to a friend!
>"All antenna systems must have a good counterpoise...." Nope, not true. Want examples? J-pole.

A dipole or a loop is an example of an antenna that doesn't need a ground plane or separate counterpoise. Is a J-pole really a good example, though? All the construction articles I've seen say to coil the coax below the J to prevent antenna currents on the feed line, which sounds to me like the antenna is "trying" to find a counterpoise to work off. Which makes sense a priori since it is a half-wave vertical at heart -- the stub section is a transmission line transformer that creates an impedance match, but that's not the same as a ground plane.


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