Even though I have yet to participate in any type of Radio Direction Finding (RDF) event, I find myself buying and building stuff for it. This time I decided to build an offset (active) attenuator as I think it’s a must need for RDF. When I was testing out my 3EL tape measure Yagi, I placed a transmitter on my property and tried to find it with a Yagi and found that it was near impossible to pinpoint the source as my radio was showing full scale and dead full quieting no matter where I went.
Since I’ve been reading a lot about fox hunting, I knew I needed an attenuator. However there are different kinds of attenuators that you can make or buy commercially. I wasn’t sure what to get at the time. I narrowed it down to the offset attenuator and the step attenuator. I went with the offset attenuator because it appears to be cheaper, easy to make and better than a step attenuator.
The attenuator that I went with was found on HomingIn.com’s Website. The article was writen by Joel Moell (K0OV) and explained in detail about the attenuator. What the offset attenuator does is “Offset” the received signal by 4MHz using a diode, oscillator and some other passive components. You are now listening to the signal away from it’s transmitting frequency. Your antenna and radio is no longer being overloaded and you’ll be able to get even closer to the signal.
It appears to be quite easy to build, even for me! So I went with it. The parts that are listen in the article are a little outdated
Here is an updated list of parts that I purchased. I usually use Mouser for components but I wasn’t satisfied with their shipping to the North East so I used Digi-Key with better results
1X,CAP CER 470PF 2KV 10% RADIAL,$0.23 ea,1286PH-ND,Digi-Key
2X,CAP CER 4700PF 50V 10% RADIAL,$0.30 ea,BC2683CT-ND,Digi-Key
2X,RES 2.2K OHM 1/4W 5% CARBON FILM,$0.10 ea,2.2KQBK-ND,Digi-Key
1X,RES 4.7K OHM 1/4W 5% CARBON FILM,$0.10 ea,4.7KQBK-ND,Digi-Key
1X,DIODE SMALL SIG 100V 200MA DO35,$0.10 ea,1N4148TACT-ND,Digi-Key
1X,OSC 4.0000 MHZ FULL SIZE,$2.49 ea,CTX774-ND,Digi-Key
1X,BNC FRONT MT RECEPT SHORT,$4.67 ea,ARF1064-ND,Digi-Key
1X,5K Audio Tape Pot,$3.49 ea,#271-1720,Radio Shack/In-Store
1X,Perf Board/PC-Board,$2.49 ea, ,Radio Shack / Instore
1X,SPST Swith,$2.49 ea, ,Radio Shack / Instore
Total project cost: Approx $8.00-$18 USD
Making Sure Everything Works
Before putting it on any type of board I wanted to make sure that It works. I never really messed around with making electronic devices from a pile of parts. I put the entire project onto a breadboard following the schematic as close as possible
Here it is being tested out on the breadboard It went together pretty easy. I used a voltage meter to make sure the correct voltage is coming out of the LM7805 regulator. I was seeing around 5V
Here is a video of it in action
Now that I know it works, it’s time to transfer the design to a more permanent home. I wanted to compact it as short as possible to get it to fit into a small PVC box. Since I had a PVC box I wanted the board to go into, I measured a piece of perf board and cut it up
After cutting the board, I laid out all the components and attached all the wiring I could from underneath the board
Here its with most of the stuff attached.
Here is the Top view of the board. I had to use some jumpers (red and green wires) to get some of the components to make contact.
The black wires you see leading away from the board are for power and the adjustable resistor. I tried to test it out at this point to make sure it works before adding the coax and other things but It turned out not to be worth doing. But I did check the circuit wiring a couple times to make sure.
Getting it jammed into the small box was going to be difficult. I knew right away that I wouldn’t be able to fit the 9V battery and the circuit board into the same compartment without using a larger box.
Here is the PVC junction box with the circuit board, switch, POT and cabled jammed into it. I had to have shave some of the flange off on the cover as the Pot is almost the same size as the Inside dimension of the box.
To solve the battery issue, I fabricated a small aluminum box that can hold a 9V battery. I drilled a small hole in the side of the PVC case to route the power cable through. In the above picture you can also see the on/off switch and adjustable pot. I mounted the switch sideways to avoid any accidental switching even though it’s still possible.
Here is the fully assembled antenna. The PVC junction box is also used to mount the grip handle. This way most of the weight is sitting on top of my hand instead of out on the boom. I also didn’t want to put any kind of electronics/metal between the reflector and driven element. Not sure if it would make a difference but I think it’s better off this way
I learned a lot while making this attenuator. The circuit was simple enough to where I can understand what is going on.
If I were to build another one, I would make some changes to make it even better. The big problem is that the coax runs from the driven element straight into the attenuator from inside the PVC. This doesn’t allow me to swap out antennas. What I would do is put a BNC connector sticking out of the PVC box and have the coax come out the boom to make the connection. I could just make an attenuator that is seperate from the antenna but that is just another bulky piece of equipment to carry around. I wouldn’t want to attach it directly to the radio because I think it would put strain on the connector that is in the radio.
You also can’t TX using this antenna. If you do, you can kiss the diode and possibly other parts goodbye. I would try to install some kind of switch that would allow me to TX but I’ll just carry an extra antenna or extra radio for now.
Hopefully it will see a lot of use.
Thanks for reading!
Jeffrey Bail (NT1K)