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 The parts that are listen in the article are a little outdated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)
Since I’ve built a ton of J-Poles and wire Antennas, I’ve wanted to build something different. I decided on a 3 element YAGI built for GMRS that is directly fed with 50ohm coax. After a couple of failed yagis and the help of another ham on QRZ.com forums, I finally built a Yagi that works! The reason I’ve chosen a Yagi built for GMRS is due to the ultra high frequency, which ends up being a small antenna. If I were to mess up (Which I did), the material cost would be low. I also wanted to use it on a GMRS repeater in the area.
The first Design I used is with a Web Site that has a Java base applet to design the Yagi, After getting all the Dimensions from the website, I went to work building the antenna. After everything was done, I learned two things. One is that my drill press does not drill straight (90 Degrees) through the tubing. The other thing is that when I hooked up the antenna to a simple SWR Meter, That didn’t work either (Pegged the Meter). At this point I got frustrated and posted my issue on QRZ.com. A Ham by the call of WB3BEL (Harry) took my dimensions (That I got from the applet) and plotted my antenna into 4NEC2 software (Like EZNEC but freeware) and it would not work for the center frequency of the GMRS Band (or any part of the GMRS band).
WB3BEL actually re-designed the antenna to where it would work so I give him credit and major thanks for help. I took his Dimensions, Modeled the antenna for fabrication and built the antenna. I Hooked up the antenna to a transceiver and SWR meter and got a 1.2:1 SWR and a 1.5:1 SWR on the outsides of the GMRS band. The Design is calculated to yield 7.5Dbi of Gain. Considering connector and cable loss (Lets say 4Dbi using 50ft RG-213 W/ 3 SO-259 Ends and a Barrel Connector) still yields gain of around 3.5Dbi which is not too bad.
Here is rendered Image of the Antenna. The elements are Insulated from the boom using plastic shoulder washers for the Reflector and director. The Driven Element is insulated using a 0.750(OD)X.375(ID)X1.5″(L) Plastic spacer. Since the elements are going THROUGH the boom, It will make the elements electrically shorter so you have to compensate for the loss by adding 0.279528″ (7.1mm) to the element to correct the effect (Boom correction). The elements are secured using #8-32 Screws screwed to the boom. The screws are also insulated from touching the boom. The screws do not make any significant changes to radiation pattern of the antenna as long as it’s insulated from (not touching) the boom. I did notice that the screws actually lowered the SWR a tad which is great.
I didn’t add a matching network to the antenna because I wanted an easy to build and assemble antenna which is the entire point of this article. The antenna is fed using RG-213 Coax with terminals soldered to the core and shield. I tried to keep everything as short as possible because this and the ring terminals effect the performance and SWR of the antenna.
Here are a couple of screen shots from the antenna software that show the Radiation Pattern and gain. Nothing special here.
Here is the calculated results for the SWR of this Yagi. Please note that it’s in the ball park. By adding screws, coax leads and the ring terminals, it could or will effect the final pattern and/or performance of the antenna.
Continue reading if you want to build this antenna.
In the past couple of weeks I picked up a couple commercial two-way radios. I’ve purchased a Motorola Maxtrac 300 (Mobile) and a XTS 3000 (Handheld).
I’ve been a fan of commercial radios since I got my hands on a HT1000 and beat it up pretty good. I’ve dropped it, thrown it, threw it into a puddle and who knows what else and they always worked out.
There are PROs and CONs about owning a commercial radio for Amateur use. One of the CONs is programing. Unlike amateur radios, the majority of commercial radios have to be programmed. With Motorola, depending on the situation, the software that is used to program these radios will end up costing as much as half (or more) of what you purchased the radio for. You could also have a local dealer program the radio. Another one of the CONs is the ability to change the frequency “On The Fly”. You can only change to a frequency (Channel) that is programmed into the radio.
One of the PROs are that you will get a radio that is built to public safety and/or military specifications. These radios can take a lot more abuse then it’s amateur counterpart. Another PRO is that these radios are built as “Part 90″ (Public safety, Private business, municipal, etc) radios. That means you can use these radios in both the LMRS and Amateur frequency blocks. Your not allow to take a amateur radio and modify it to transmit in the LMRS. So if you have a job that uses radios in the LMRS blocks or you want a radio that can do both GMRS/UHF (Or MURS/VHF) then you will have a radio that will possibly cover all 3 possibilities.
This is the Maxtrac 300 I got on e-bay for around $100. It was being advertised as a 430-470mhz split which is rare but when I hooked it up to the computer, it was the 450-470mhz split.
Just a note for anyone that is looking to buy this or it’s GM300 brother on eBay. Make sure the auction has the Model number in it. For example, the model number D34MJA7JA5AK means that’s it’s a 10-25w (2nd Number), UHF (3rd # ) Conventional (Numbers 4,5,6 and 7[MJA7]) 32 Channel (8th # [J]) A5 HEAD (9,10th #) Revision K (Last Digit).
So now you know your getting 10-25 watt 32 Channel UHF radio. However these types of radios have 2 splits per band. On Uhf you could get getting a 403-430 or 449-470 split. I’ve read that there is a 430-470 Split RF but I have yet to see it. The only ways to tell what split the radio is, by looking at the radio in the RSS (programing) software or actually opening up the radio to see what the part number is on the RF Board. So be careful if your looking for a Ham band Maxtrac.
This is my Motorola XTS 3000 UHF Handheld radio. I purchased this item because the “But It Now” price was just where I wanted it. Another reason is that it can decode/encode P25 Digital. Always wanted to mess around with it and now I have the chance. Only issue is that there is only two P25 systems for ham radio in the state of Massachusetts and they are both located out of range. If there is enough interest in the area, I would like to setup a repeater capable of P25. We’ll see…
Thanks for reading!
Today I gave my self a Christmas present. Before this purchase I did not own a working VHF/UHF Radio. I have a Kenwood TH-78a that I love. It just needs a new battery pack to get me going. I went out following a snow storm and drove 40mi to Lentini Communcations in Berlin CT. which is my nearest “HAM” radio store and purchased the WOUXUN KG-UVD1P Dual Band (144/440) Transceiver. I was a bit skeptical about buying a Chinese branded ham radio that is not one of the big three (Yaesu, Kenwood and ICOM), but I figured the price is right ($130 w/ programing cable) and QST magazine gave it a good review.
First impression was not bad. The box looks nice and everything was neatly packaged. If I were to be over critical, I would say that the programing/usb driver CD’s should be regular sized instead of the Mini-CD that was given to me. I like that the battery is fully charged and it includes a drop in charger which I think should be a standard with any handheld that is purchased. The manual just gives the basics and has a lot of engrish so for those who depend on reading a manual rather than trial and error to operate a radio will find it a tad difficult to follow. You can also tell the radio is designed for general public use and not for amateur radio but that doesn’t phase me considering I’ve owned Motorola radios which I think they are harder to program.
After attaching the battery pack and the antenna, I powered on the unit to hear a voice speaking to me. It took me around 5min to look around, input a frequency, input a tone, set the offset, assign a offset and start talking on a repeater 50 Miles away. I wasn’t crystal clear but I can hear the other station and the other station could make me out.
I am going to start off with the cons
- Antenna connection is reversed (SMA FEMALE ON THE ANTENNA, SMA MALE ON THE RADIO)
- Not true Dual VFO. You can monitor two freq’s but locks out the other on activity.
- Hard to program through keypad
- S-Meter doesn’t seem accurate. I was barley getting a repeater and the S-Meter was showing full bars
- Can’t program both side keys
- PRICE PRICE PRICE!!! $130.00 US for everything
- Desktop Charger!
- Great receive
- Great Transmit
- Long lasting battery (7.4v 1300 mAh Li-Ion)
- Decent Size (2-1/4″ Wide X 4-1/8″ Tall X 1-1/2″ Thick)
As of right now I would recommend this to anyone. However I strongly suggest if you purchase this radio to also get the programing cable and software (you can download the software from their website) .