Hi Thomas, it's sad that the level of man made noise where most of us live means really neat antennas like the TCM are less rewarding to use.sdrom33 wrote:Hi Phil,
<snip> Far from being offended, but really thrilled, I understand the problems with the TCM exercise: it is not small, has no gain and is more susceptible to noise intake than other antennas. You wrote about a "small" 20 m loop, but how small is small? May I ask you to expand a little on this antenna? What would happen should I wish to use it on 40 m as well?
Don't know if you've ever read much by Cebik on antennas, he was my hero, I've modelled and built and used dozens of his antennas over the years. When it came to simple and effective HF antennas he wasn't all that excited by large wire loops as he considered using them mostly on their fundamental frequencies where they primarily used for NVIS. Plus his focus was both Tx and Rx on the ham bands which is quite different from todays renewed interest in wide band listening that has blossomed with SDR devices.
His work is absolutely marvellous though and makes great reading, it used to be behind a paywall until recently:
Large wire loops here; there are other articles about them too but this should answer a lot of your questions:
A "large" wire loop as opposed to a compact HF loop is simply the largest area you can accomodate within your backyard's limitations using a wire. Making it delta shaped reduces the remote mounting points to two and causes it to be more omnidirectional than a geometric square "quad" shape.You wrote about a "small" 20 m loop, but how small is small? May I ask you to expand a little on this antenna? What would happen should I wish to use it on 40 m as well?
For best performance on the ham bands, because of their harmonic relationship, choose a total wire length that is equal to one wavelength on the lowest frequency you'd like to work on. Eg. resonance on 3.55MHz would ensure that the loop has as low a reactance and return loss as possible on 7.1MHz, 10.65MHz, 14.2MHz, 17.75MHz, 21.3MHz, 24.85MHz, 28.4MHz and so on.
On other frequencies regardless of resonance you just accept whatever characteristics your backyard causes your loop to have, in the same way we accept the limitations of so-called long wires and whatever. There is a key difference with the large loop though, they tend to have a very much more even impedance and low reactance compared to non loops, with the added very marked advantage of lower noise / balanced feedline.
There has in the past been much debate about whether loops are somehow more immune to noise than non-loops; all I can say is that at my location there is no comparison at all, all my many many best efforts with very long wires, really well built and sited inverted L antennas (a favourite of Cebik) and even dipoles have resulted in incurable and pretty much unworkable noise. Travelling outback Australia is the only place I've found that inverted Ls and verticals really come into their own, and I've done that extensively for two years living in a 4WD truck, modelling and dreaming antennas all the way.
Bottom line: try it, if it doesn't work out for you then hopefully you don't have the means to fly over here and strangle me.
BTW, these are of an older loop taken some time ago before Steve Andrew's beaut spectrum analyser software, at least you can see how you don't get unworkable high impedances on even harmonics like you do on everything else.
Together these pics cover 1.8MHz and up.