Low Noise Terminated Antennas
I think I’m infatuated with the idea of low noise reception in the HF bands.
My interest started 30 years ago when designing and building an ionosonde for use in Antarctica. I had the opportunity to build an entire receiver from scratch (and more importantly, with someone else’s money.) Now, there are some tricks you can pull to drop the noise of an HF receiver, and there are still a few ideas I’d like to test… but the front end begins at the antenna, and if you can improve the signal-to-noise there, you start out ahead of the game.
I became inspired again in recent months when a vertical antenna I was testing became shorted in an unusual way (involving water) and became low noise but with decent signal reception (and horrible transmission results.)
One antenna that’s popular with SWLers is the TTFD (or T2FD), the tilted terminated folded dipole. It’s essentially a folded dipole, but with greater distance between the folded elements and a termination at the far end from the feed. It’s very broad band and is a standing wave design somewhat like a rhombic where induced opposing currents terminate, rather than reflect. Of course, as a result of the lower feed point currents, the feed has a high resistance, and it’s terminator should be roughly match that. 400-600 ohms is reasonable, and a 6:1 to 9:1 balan comes in handy if you want to use coax. Also, watch out: there’s a lot of misinformation about the TTFD and erroneous construction details. This TTFD document by Rob Wagner seems pretty good, and “Buck” Rogers K4ABT has the best T2FD diagram (he sells the TTFD as a kit.) John Conover’s T2FD page also seems decent.
Tonight, I came across another similar design, the K9AY Terminated Loop by Gary Breed. It’s been getting some rave reviews because it’s much smaller than traditional low band loops, sparing you a lot of real estate. You only need one fairly short support (like a tree), and the entire loop can fit in a 15 foot radius. In addition, when built smartly you can make it four way directionally switchable. A nice feat for a small, ground level, fixed position low band antenna. The front-to-back ratio can be as much as high 40db. Quite remarkable really.
The theory of the K9AY (according to Gary) is that the E and H fields induce voltage and currents that reinforce on the front side, but will cancel when received on the backside. That’s probably due to the oddly placed feed and termination points, which are very near the grounded base and are connected to ground. And, it’s also a standing wave type of antenna, but with an interesting twist in its geometry relative to its feed, termination, and ground.
If what this antenna claims to do is true, it might be superior to the TTFD in several ways.
Somehow I’ve got to find the time one of these weekends to build both of these interesting designs.