Base Station Hands-Free TX

It doesn’t take much time in the ham hobby to hear of some very expensive pieces of equipment.  One item in particular peaked my interest very quickly when I heard that you could have a QSO hands free!  You mean I don’t have to hold the mic and push this little button to talk on the radio at home?  Indeed, this was true, or at least somewhat true.  A mic still needs to be held and a button still needs to be pressed.  Exactly how that happens can be the creative part!

Let me go ahead and cover a subject before I get a page full of comments here.  I am well aware of VOX.  I personally find that VOX works better on SSB and in an environment with little to no background noise.  For my purposes, this wasn’t my ideal situation.  My issues/opinions with VOX are as follows.  It is limited to HF and not FM repeater QSO’s.  Adjusting the VOX settings were a little difficult to set correctly.  I had to refrain from speaking, laughing, coughing, sneezing or making any comments verbally that I didn’t want immediately transmitted over the air.  I saw very quickly that this was going to get me in trouble! Ha!  I very well may find later on down the road where VOX has it’s place in my ham shack, but for this project, VOX was a future option but not a currently used feature.

If you do not yet know what VOX is, we’ll cover that toward the end.

With that being addressed, let’s get back to the project at hand.  What could I do to take the place of the mic and the button that was used to transmit?  There are several commercial options available to the ham radio operator; head sets, boom mics, foot switches, desk mics… so on and so forth.  Look at any of these and you will very quickly learn that none of them are cheap!  There is one very common mic company (that will go unnamed for now) for commercial and amateur use that could provide a very nice setup for your rig for a few hundred bucks.  I wasn’t about to fork out that much money just to be lazy behind my radio console but I was eager to find out what I was able to home brew with as little as possible.  I personally liked the idea of the boom mic with a foot switch… a mic held in place for me and the transmit button on the floor.   And so was my goal now set.

I needed to learn about this radio and how the existing mic works before I was able to build my own.  I knew that the audio and the command for transmit would have to be sent to the radio via the mic cable.  With a little investigating on the internet, I found the schematics for my particular radio and stock mic. On my particular radio (Yaseu 857-D), the mic plugs into the radio with an 8 pin RJ-59 connector.  2 pins (+ and -) carry the audio and 2 different pins (+ and -) are for the transmit.  The button on the mic creates a closed circuit between the two different pins and that puts the radio in transmit.  The audio then travels through the 2 audio contacts and the message is delivered through the radio.  With this bit of information, I knew that I needed a mic for the audio and a switch that could create a closed circuit on demand.  Now I just needed to search for my supplies and go from there.

Photo Dec 19, 13 23 24I stopped by my local mom and pop speaker repair center and inquired about a few items needed for the project.  Knowing a little about what he would have, I only asked this particular gentleman for a used mic and foot switch.  He asked about my purpose so I explained.  He said that he didn’t have anything in the price range I was happy with but he said that he had something else for me.  He went to the back and rummaged around for a few minutes when he returned with the diamond in the rough.  He had an old metal boom with a mic holder on the end.  The paint was worn but all of the springs and other hardware was in good shape and present.  I tried not to act too excited to drive the price up but I asked him what his wanted for this piece of equipment.  He pondered for a moment and came up with the price of 5 bucks.  I probably had the cash in his hands before he finished his answer.

I then went to a local music store and asked about the mic and foot switch.  They had a fancy karaoke XLR mic for 5 bucks and a guitar pedal for another 5 bucks.  I took them both and headed to the house.

Technical note:   XLR is a type of connector primarily used in professional audio.  It is most commonly associated with a balanced audio connection.  XLR mics usually have 3 pins (positive polarity, negative polarity and chassis ground) that go to sound boards for mixing.  

Going back to the radio schematics, the radio only accepts 2 wires (unbalanced) for audio.  I discovered that by rewiring the soundboard side of the audio cable, you could convert balanced audio (3 wires) to unbalanced audio (2 wires).  This is what I needed to go to the radio.  Let’s look at the foot switch before we put it all together.

So I learned through testing with an ohm meter that the PTT (push to talk) button on the stock mic closed the circuit to put the radio in transmit.  With this guitar pedal, all I needed was for the circuit to be normally open and then closed when pressed.  I quickly learned that the pedal I picked up for 5 bucks was just the exact opposite.  It was normally closed and opened when pressed.  So I returned it to the store and looked for another solution.

Photo Dec 19, 11 16 35I found that they make other pedals (i.e. electric piano sustain pedal) that are switchable for their operation.  They were labeled as a polarity switch which didn’t exactly seem appropriate to me but it fixed my problem.  On one setting, the circuit was normally closed and on the other setting, the circuit was normally open.  I needed the later of the two so I spend the extra 5 bucks and picked up a new “piano sustain pedal” with polarity switch.  This came with a 1/4 inch mono plug on the end that led me to an idea of how to put all of this together.

So, now I have a mic and I have a momentary PTT switch.  I looked at the mic connector on the radio and it resembled a cat5 network cable connector.  I tried an old network cable (RJ59) that I had laying around and it fit!  Not only did it fit but it had the 8 contacts on it with 8 wires.  Now I just needed to bring it all together.

Radio shack immediately came to mind when I thought about audio jacks and other project type materials.  I picked up a small project box, (2) 1/4 audio jacks and a momentary switch.  I mounted the 1/4 audio jacks opposite of each other on the box and mounted the momentary switch on one broad side and drilled a small hole on the other.  One 1/4 audio jack would be for my PTT pedal.  The other 1/4 audio jack would be for my mic and the small hole would be for the network cable that would make the connections to go to the radio.

What is the momentary switch for then?  Well… that’s a good question.  I knew that the project box that I was building would be the junction for all of the different elements to come together.  I wanted yet another way to put the radio in transmit other than the pedal.  The pedal would fulfill my purpose but I wanted more options.  After all, a desk mic usually has a  short boom for the mic and then has a  PTT switch on the base that allows for the one handed transmit while speaking into the mic.  So, as easy as it would be to integrate, I bought a normally open switch that closed the circuit when pressed.  This had to match the pedal because it was essentially preforming the same action.

This is a schematic and this is what mine looks like.

Photo Dec 19, 11 31 26

I identified which 2 wires needed to go to the mic jack ( + pin 4, – pin 5) and attached them appropriately (positive to the tip) .  I identified which 2 wires were part of the PTT circuit (- pin 2, + pin 3) and essentially hooked them up in parallel so that either one of the switches could close the circuit independently of one another.  (The polarity doesn’t matter on this connection.  It’s either opened or closed.)  So what I have now created is the junction of the 2 PTT’s, the audio input and the cable that goes to the radio.  The only missing link is the connecting the audio source (microphone) to the junction box.

Origionally, I found a way to wire the 1/4 inch plug to the audio cable that would convert the balanced audio coming from the mic to the unbalanced audio source. (Connect pin 1 & 3 to the sleeve and 2 to the tip)  You can also purchase a pre-made cable that does this as well.  As well as this worked, my signal reports were generally good but included a little bit of noise on the signal.  It was emphasized that it wasn’t anything that distracted from the QSO but noticeable.  Investigating the difference between balanced and unbalanced, it was clear that the unbalanced option allowed for more unwanted noise to enter the signal and get sent to the radio.  Ovbiously this was not a significant problem in my case but I was in search of an alternative solution.  Also worth noting, with any more small amounts of RFI around or other sources of noise, this could easily be a bigger problem in the future or in other environments.

After consulting with some friends that had experience in commercial radio, the idea was given to process the audio signal through a small mixing board and then to the radio.  This would allow me to keep the audio balanced for as much as possible.

011531f349c2081154a9f7d81bf32d2813ee666b33 I picked up a small 5 channel mixing board from a local retail store for $45.  I could have probably waited around on the use market for a cheaper one that would have met my needs in the neighborhood of $25 but I was eager to see if it would work as well as I had hoped.  The mixing board would process the audio and then output the audio with 1/4 plug to the project box with a clean signal.  (As a bonus, depending on the mixing board, you can also adjust the treble, mid and bass as well as adding effects if you so desire.  (Just be careful not to sound too much like you belong more on 11 meters than you do 10!)  I chose to utilize the EQ on my audio but that is as far as I went.)  This setup also allows me to use more than one mic input and even an computer audio feed for replaying recorded audio if needed.  (Very useful to allow radio operators hear their own signal if they are trying to diagnose a problem.)  The mixing board could most definitely be left out of the equation but it is a viable option.

After you decide how to route the audio to the project box, that completes the project!  You have your audio input, your footswitch PTT with the option of the PTT on the box and then the network cable that connects all of this to the radio.  As an assembly note, I used a grommet to go around the network cable and protect the cable from the edges of the box.  It also provides just enough pressure to hold the cable to prevent it from pulling out of the box.

During use of the hands free station, I discovered that it worked flawlessly on VHF/UHF.  On HF, however, I was getting RF into the mixer.  It was just heavy static that I could hear through the monitor outputs and I could see my power meter maxing out even though I wasn’t speaking on SSB.  I placed ferrite chokes on the audio cable and on the CAT cable, both at the box and the problem was resolved.


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