I guess it's because I'm fairly young (30-40ish) and I grew up in the 80's and 90's, but I can't hear the word "transformers" without following it up by singing in a somewhat robotic voice - "robots in disguise" (Here is link to a sound clip from the opening credits of the cartoon if you don't know what I'm talking about.)
As far as HAM radio goes, I'm actually talking about RF transformers, not the robots. The one I recently bought looks almost as cool (maybe?) as a robot.
As far as HAM radio goes, I'm actually talking about RF transformers, not the robots. The one I recently bought looks almost as cool (maybe?) as a robot.
The word balun get's tossed around a lot in ham radio, but for my car, I needed an unun. For other newbies like myself out there, a balun matches a balanced antenna (like a dipole) to something like an unbalanced transmission line (like coax.) Because most mobile antenna's are some variant of a vertical (1/4 wave, 1/2 wave, 5/8 wave, etc.) mobile antenna's are usually unbalanced, so if you need to "make a match" like is often needed on HF, an unun is in order. I currently swap hamsticks in and out for my Mobile HF, but on the lower bands I couldn't get a match.
I ended up buying the unun transformer MFJ makes. I mounted it under the parcel shelf in the trunk of the car, so now it is a simple matter of reaching under and twisting the knob to the location I identified during tuning of the antenna, along with popping on the corrosponding Hamstick. While I was messing around back there, I also finally put the ferrite beads on the choke that is wound right where the antenna enters the car. Here are some pictures of what I did:
The only antenna that I had time to tune up (so far) with the the new unun in place was the 40 meter Hamstick. Before, the lowest SWR I could get with this antenna was about 1.9:1 which meant I could only use it on the one frequency I tuned it to - not much fun. Now with the unun in place, the lowest SWR is just over 1:1, and I can cover a pretty decent chunk of the voice portion of 40 meters - yay!
I did notice something interesting though - the directions that come from MFJ tell you to tune the antenna for lowest SWR with the unun out of the line first, then put the unun in the line, and switch through the settings until the lowest SWR is found. My gut told me to do something different, but I followed their directions. Without the unun in place, I tuned the antenna to the lowest SWR I could get, at about the center of the voice part of the band (7.212). This was my 1.9:1 SWR. I then put the unun in line, but the lowest SWR I could find switching through was about 3:1. Luckily, my antenna analyzer (the RigExpert AA-50) does a graphic sweep, so I could see that there was an SWR dip down to about 1:1 just slightly higher in frequency. Once I retuned the antenna again, I was all set. This led me to believe that the directions that come with this unit (the MFJ-907) should follow this format:
1) Tune antenna for lowest SWR without the unun in the line
2) Put the unun in line, and switch through the settings until you find the new lowest SWR
3) Re-Tune the antenna for lowest SWR at the desired frequency
With all that being said, here was my though - before I put the unun in line, should I really be tuning my antenna for lowest SWR, or should I be tuning it for resonance (X=0)? With my 40 meter Hamstick, lowest SWR and resonance are actaully pretty far from each other. I don't have the numbers in front of me at the moment, but at 1.9:1 lowest SWR, the reactive part of the impedance was relatively high. When I scan up and down in frequency on the analyzer looking for the point where the load is all resistance (maybe not all resistance, but as close as can be expected), there was a point higher in frequency that was something over 100 Ohms, pure resistance (so higher than 2:1 SWR) and there was a point much lower in frequency that was about 8 Ohms pure resistance ( higher than 6:1 SWR).
So now I look to the Elmer's of the world....should I play around a little, and re-tune based on resonance, and then put the unun in line, or should I just be happy that I now have an antenna that "works" on 40 meters in my car? If I do re-tune based on resonance, should I start from the 6:1 SWR point, where the antenna is longer, or should I start from the just over 2:1 point where the antenna is shorter?
Even if I don't get answers to my questions, I'll probably play around with all the options, and just see what it does. I might even download the data from my antenna analyzer and share it here, so that everyone can look at it with me. Right now thought, it's COLD outside, and I can't really tune a tall HF antenna with the car in my garage, so it might have to wait for a warmer day - at least one above freezing.
I did notice something interesting though - the directions that come from MFJ tell you to tune the antenna for lowest SWR with the unun out of the line first, then put the unun in the line, and switch through the settings until the lowest SWR is found. My gut told me to do something different, but I followed their directions. Without the unun in place, I tuned the antenna to the lowest SWR I could get, at about the center of the voice part of the band (7.212). This was my 1.9:1 SWR. I then put the unun in line, but the lowest SWR I could find switching through was about 3:1. Luckily, my antenna analyzer (the RigExpert AA-50) does a graphic sweep, so I could see that there was an SWR dip down to about 1:1 just slightly higher in frequency. Once I retuned the antenna again, I was all set. This led me to believe that the directions that come with this unit (the MFJ-907) should follow this format:
1) Tune antenna for lowest SWR without the unun in the line
2) Put the unun in line, and switch through the settings until you find the new lowest SWR
3) Re-Tune the antenna for lowest SWR at the desired frequency
With all that being said, here was my though - before I put the unun in line, should I really be tuning my antenna for lowest SWR, or should I be tuning it for resonance (X=0)? With my 40 meter Hamstick, lowest SWR and resonance are actaully pretty far from each other. I don't have the numbers in front of me at the moment, but at 1.9:1 lowest SWR, the reactive part of the impedance was relatively high. When I scan up and down in frequency on the analyzer looking for the point where the load is all resistance (maybe not all resistance, but as close as can be expected), there was a point higher in frequency that was something over 100 Ohms, pure resistance (so higher than 2:1 SWR) and there was a point much lower in frequency that was about 8 Ohms pure resistance ( higher than 6:1 SWR).
So now I look to the Elmer's of the world....should I play around a little, and re-tune based on resonance, and then put the unun in line, or should I just be happy that I now have an antenna that "works" on 40 meters in my car? If I do re-tune based on resonance, should I start from the 6:1 SWR point, where the antenna is longer, or should I start from the just over 2:1 point where the antenna is shorter?
Even if I don't get answers to my questions, I'll probably play around with all the options, and just see what it does. I might even download the data from my antenna analyzer and share it here, so that everyone can look at it with me. Right now thought, it's COLD outside, and I can't really tune a tall HF antenna with the car in my garage, so it might have to wait for a warmer day - at least one above freezing.
RSS Feed
