Right now I am contemplating what I'm going to do for tuning my outdoor vertical AM antenna. I had an inverted L in the past that fell down and putting it up with a crossbow was really hard, so I don't think I'll try that again.
Anyway my question is about which route to take for tuning the antenna. There are several options:
1. I have one high power ham antenna tuner. I could put it outside in a weatherproof box at the base of the antenna. I think this would work well, but it would be hard to tune up going indoors/outdoors a million times to read the SWR. Maybe I could just put the SWR meter in the box as well, but I've come to find that my SWR at the antenna feedpoint != good SWR at the end of the coax inside.
2. Maybe I could just use some really low loss cable (I've got some LMR400) that should be long enough and then just have the tuner inside and forget about anything outdoors. Would the losses still be too significant?
3. This is the hardest, but I could put a "tuner" both outdoors and indoors. If I do this, can I just use a loading coil on the base of the antenna, or will I need a capacitor or ground shunt? (Or anything else besides the coil outside?)
What do you think? The antenna will probably be only about 20ft high with as much capacitance hat wires as I can add drooping down at a 45 degree angle.
Why even use the ham antenna tuner? I would think using a standard loading coil wound around an insulator and tapping up on it (click me) to find best SWR and resonance would be far easier, efficient, and resilient to weather changes, or the usual L-network coil/capacitor tuner but that would require somewhat better weather proofing for the capacitor. If a single tapped coil is a pain in the butt you can always wind the loading coil and "hairpin" SWR match coil as separates.
Lots of info out there if you look up Part 15 AM antennas and how those low power guys are doing it with 10' antennas. Another idea that comes to mind for your 20' antenna is making some of it helical near the top or add top coil loading, then the tuner becomes simpler and less power wasted in it. Either way, always do the tuning at the antenna end or else you will be adding in other complicated variables if done at the transmit end.
I would measure SWR first at the antenna and then at the transmitter. If there is a significant difference then something else is at play, possibly the coax run acting as an added ground radial? Standing waves along the coax run can give you a different reading depending on coax length which will give you an erroneous reading by subtracting or adding to the real reading when measured at the transmit end. Measuring at the antenna end will prevent that until you get the SWR down to an acceptable level, which I should mention with such a shortened antenna may be next to impossible. You may have to live with a higher than normal SWR at these low frequencies and compromise antenna systems, that doesn't mean it's not working well. As long as your transmitter is running cool and the signal is getting out good, I'd call it a day.
For the coax itself since we're talking frequencies lower than 1.7MHz I would not worry about using expensive low loss type unless you have it already. At these frequencies even cheap stuff will be low loss. Just for fun even RG-58 which is normally terrible for long runs at high frequencies will only lose 0.4db at 100 feet of cable run at 1MHz, no one listening would ever notice that tiny loss in strength
Why even use the ham antenna tuner? I would think using a standard loading coil wound around an insulator and tapping up on it (click me) to find best SWR and resonance would be far easier, efficient, and resilient to weather changes, or the usual L-network coil/capacitor tuner but that would require somewhat better weather proofing for the capacitor. If a single tapped coil is a pain in the butt you can always wind the loading coil and "hairpin" SWR match coil as separates.
Lots of info out there if you look up Part 15 AM antennas and how those low power guys are doing it with 10' antennas. Another idea that comes to mind for your 20' antenna is making some of it helical near the top or add top coil loading, then the tuner becomes simpler and less power wasted in it. Either way, always do the tuning at the antenna end or else you will be adding in other complicated variables if done at the transmit end.
I would measure SWR first at the antenna and then at the transmitter. If there is a significant difference then something else is at play, possibly the coax run acting as an added ground radial? Standing waves along the coax run can give you a different reading depending on coax length which will give you an erroneous reading by subtracting or adding to the real reading when measured at the transmit end. Measuring at the antenna end will prevent that until you get the SWR down to an acceptable level, which I should mention with such a shortened antenna may be next to impossible. You may have to live with a higher than normal SWR at these low frequencies and compromise antenna systems, that doesn't mean it's not working well. As long as your transmitter is running cool and the signal is getting out good, I'd call it a day.
For the coax itself since we're talking frequencies lower than 1.7MHz I would not worry about using expensive low loss type unless you have it already. At these frequencies even cheap stuff will be low loss. Just for fun even RG-58 which is normally terrible for long runs at high frequencies will only lose 0.4db at 100 feet of cable run at 1MHz, no one listening would ever notice that tiny loss in strength
Thanks for the reply!
So if a single loading coil would be enough then that doesn't sound too hard. I wonder if there are any roller inductors that have a high enough inductance for this. That would be ideal because I could get the tuning exact and be able to easily change it. What would be the nH roughly that I would need? Maybe I could have a base fixed coil and then a roller to tweak it if they don't make the rollers high enough.
I also have 50ft of LMR400 that I use for FM usually. Not sure it will reach, but it might. Since this is so low loss, then maybe I can just keep the tuner inside and do nothing with the base of the antenna.
Either way, I got board and tried loading up the metal roof on my barn. It might not be a great antenna, but I figure a large high mental object is probably going to work OK. It's not exactly over the ground plane, but I was able to tune it. I did the experiment with 1710khz and then went on my computer in the barn and then realized the mouse was barely working and the internet didn't work LOL. So I went in the house instead and went on websdr.org and tried to listen to my signals from SDRs. Wasn't successful. I decided to retune to 1720khz and then heard myself in PA (about 250miles away) It was a very low signal though. I had to adjust the bandwidth to 2khz to hear something. I'm very impressed that it worked at all though.
Yeah I noticed the same issue with a laptop I use only for Stereo Tool as my AM front end audio processor, flip on the transmitter and the touchpad becomes unresponsive or super jittery if within 10' of the ATU. Starting to think I'm going to have to run the laptop in a faraday cage lol.
Barn roof is a new one for me. I've heard of people loading up metal gutters and ladders. Guessing your roof was acting as one gigantic capacitance hat and the wire going to it as the radiator or maybe some mix of the two. Bet that can't be plotted in antenna analyzing software
As far as the loading coil there is no harm in using a smaller roller inductor in series with a larger fixed inductor to get the value you need. That's basically how I have my antenna setup right now, inverted "L" with top loading inductor before the horizontal wire section to get near the frequency I need, then a roller inductor at the base to narrow it in to resonance, that way most of the inductance is near the top of my somewhat shortened antenna to push antenna current higher up the wire rather than all at the base. Of course you can do all of this at the base too if that's more practical for the setup.
If roller inductors aren't available it's not too hard to build a variometer which will work identically. Bare copper wire spread out over PVC as a coil with a sliding bar or even a metal hose clamp to clamp on the tap point works in a pinch too.
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It was a very low signal though. I had to adjust the bandwidth to 2khz to hear something. I'm very impressed that it worked at all though.