This is a blog sort of post, things I'm learning along the way.
Inverted L 1/4 wave (monopole) antennas really are the optimal antenna for 160m ham bands when space doesn't permit the full 130'+ tower. Obviously for medium wave pirates the same holds true. It's basically a full sized tower antenna, just bent sideways at some point and vertical radiation suffers from that.
I used an L on 1710kHz for some time but base loaded it because I couldn't fit the entire wire vertically and horizontally the way that would be best. We only have so many trees and supports after all. Radials were few and small too and to be honest it sucked. Maybe a mile or two on a good night with 5-10 watts.
What I am using now is actually working far better than expected, put up 30' of wire to a pole made of conduit pipe pieces bolted together, largest at bottom to smallest at top to make a telescoping mast (wire insulated from it). Some wire off of that to a pulley and an insulator made of PVC pipe with the bend of the inverted L through it. This runs horizontally for the rest of the length out to a tree. I don't know the exact amount of wire anymore, but it's somewhere around 120' of wire total for vertical and horizontal. Still too low to resonate on 1710kHz or even the "top band" ham band right?
Well that can be solved by putting a loading coil near the top of the vertical section. I put mine right after the vertical part through the pulley on the horizontal wire, makes for taking down the wire far easier to modify it. Some 60 turns of wire around 1-1/2" PVC, bolts through each end to tie the antenna wire around inside as supports, and tied electrically slightly outside of the coil and PVC with electric tape. Makes a strong piece and easy to change windings later. By the way, the higher up the antenna the loading coil, the more turns needed to resonate it. I needed double the turns at height than I did at the base. This is normal but something a lot of people don't expect until experiencing it first hand winding, and rewinding, and winding again and so on, it can be tedious.
Base loading sucks as my previous L proved and here is why.. What is the point of the loading coil? Two things, it resonates the antenna to medium wave pirate broadcasting frequencies, it also raises the antenna current to the top of the vertical section of wire IF the loading coil is near the top which is a huge benefit as the horizontal part really doesn't get much signal out contrary to many peoples beliefs, it at most raises high angle radiation which isn't of much use for us. Loading the antenna near the base is wasting RF in the loading coil and focusing more RF current near the base, which often for most of us is near the ground. In fact the inverted L is only slightly more efficient than a sloper, and both more than a dipole at 1710kHz unless you have the ability to get a dipole up 1/4 wavelength, which is around 100'+. So obviously for short covert antennas, nothing beats an inverted L for medium wave!
With a single watt of power into this antenna I am able to get a signal out to a *pocket radio* for miles with a mostly solid signal audio wise, anywhere between 1-3. That's enough to cover my small down. On a car radio you can easily multiply that by a few numbers. That's only 3x the power of part 15 regulations in the US. It's obviously not hard to get a watt up to 10 or more, so you can imagine covering a few towns is easy this way, but of course medium wave has 1/10 the listeners of the FM broadcast band. Still doesn't make medium wave AM less fun. In fact I prefer it because it's harder to track down for the FCC and carries farther at night in the X band.
Now the other half of the antenna, this is a monopole 1/4 wave setup after all. That means the other 1/4 wave needs to be there too, one half needs to see the other half at RF, like a dipole leg needs to see it's other side to radiate. This means lots... LOTS of radial wires. They need not be much longer than the vertical section but that's speculation. Anything from 1/8 to 1/4 wavelength and around dozens of them around the antenna base going out at the ground level. Sounds terrible, but start with a few and work up from there. My setup has 4 90' some radials so far made from some Cat 5 wire an installer left behind by a happy accident all spliced up and twisted together then laid out under my trailer. The rest is running out to random things like the electrical ground box rod, some metal fencing, so on. Whatever is close to ground and metal. Copper water pipes work too if you can find one, and may travel good distance from your home.
Next up loading.. If you can't have the full inverted L 1/4 wave antenna like in my case and need to use the loading coil near the top of the vertical section, you will want to leave it slightly longer or shorter, and tune the antenna at the base still with either capacitive or inductive loading respectively. I use inductive loading (think loading coils on whips), but only enough to tune up, leave the rest of the loading coil at the top of the vertical section for radiation efficiency.
Finally.. Height matters, your groundwave and skip is going to be mostly vertical on AM. Horizontal is mostly useless besides skywave. So get your L as high vertically as possible, tune it with a current meter or SWR meter or both. Higher SWR on these frequencies might not be a bad thing since that tells you that your ground system might be optimal and SWR can easily trick you into thinking it's not. Why though? Because these antennas have a feed impedance of anywhere between 10 to 50 ohms, and lower feed impedance actually means the ground system works better to reflect the other half of the antenna meaning better efficiency and signal strength! I'd stick to a current meter at anything below 1.7MHz simply because it is easier to see your antenna working. SWR can be lowered by a "hairpin" coil across the coax feed, but if your transmitter is happy, don't worry. If not you can use a current balun at the coax end just like we use with FM and J-Pole antennas, but at AM medium wave you are going to need ferrite chokes since the frequency is so low.
Don't fuss with dipoles on AM. I know a medium wave pirate that actually got out multiple states here in the US using 1kw on AM, but I still feel it was excessive given that the same signal strength could have got out using an L or T antenna with far less power. Dipoles are cloud burners at such low frequencies so don't waste your time unless you have serious height to put them up. Besides vertical polarization has been the standard for AM broadcasters forever now. Horizontal is just wasting power IMHO.
I will fill in more text here soon when I play with this setup more.
Will enjoy any comments here. FM broadcast antennas are fairly simple compared to AM broadcast antennas. I feel the latter is an area that needs to be covered better in the pirate community, especially as more FMers fill the dial, AM broadcast is a viable option for DX work like shortwave. Love to hear more of you on 1710kHz besides The Big Q and Undercover Radio and the local TIS and Part 15 stations. Punch the end of the AM dial guys.
Pictures of my loading coil, aerial setup and other stuff to come soon. Waiting until I can get another 10' pole support section before taking the inverted L back down for pics and extending it up.
Like yourself I had an inverted L loaded at the base only, which worked poorly, until i discovered an article that mentioned splitting the loading coil, 50% at bottom, 50% at top, i thought why not give it a go, and of course it works a charm. The addition of some chicken wire below the vertical section also played a part in the substantial signal increase.
Currently running a single fet, 40 watt home brew tx.
Looking forward to those pictures of your antenna.
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