Helix aerial for mediumwave?

[ChinaCat558]

I'm quite new to all this, I'm particularly interested in mediumwave AM because I'm probably too far out in the sticks for FM to be worthwhile. I'm wondering if it's possible to build a helix aerial for MW broadcast frequencies, the idea is that the spiral shape uses less height and adds inductance to bring the resonant frequency way down. I've only ever heard of these used at much higher frequencies though, is there any reason it wouldn't work at the top end of the MW band? Wiki says that helix aerials can bring the height down to 4-15% of the wavelength while behaving a bit like a quarter-wave monopole, bringing the ~160' full-size quarter-wave aerial for 1512 kHz down to about 30' sounds a bit too good to be true though.

I'm guessing it would have quite a narrow bandwidth and need a decent set of ground radials, if such a thing could even work at all. Has anyone tried something like this and if not, what physics am I missing? If so, has anyone got a formula I can throw the parameters in and get y number of turns over x length for z carrier frequency and that sort of thing? Cheers in advance!

[Albert H]

I've used helicals a few times, with varying degrees of success. The "pseudo monopole" can be made to work, but you need a lot of ground mat. Do not expect to ever achieve a 50R resistive match - you're going to be building an ATU! As you shorten the radiator, the bandwidth reduces, so you have to add a capacity "hat", and reduce the "Q" of the ATU. There is a design for a 1.7MHz dual - helical in Radcom "Technical Topics" that uses two opposed helicals as a pseudo - vertical dipole. It was in the January 1991 edition. For 1.7 MHz, it's only 4m long. Scaled up to 1.1 MHz, it would be about 6.5m long.....

Medium Wave transmitters are quite easy these days, using big FETS for the output, and simple CMOS synthesiser circuits to obviate crystal cutting. 4.608MHz is easy to get (Farnell, 40p), and simply divides down to 9kHz for the channels - divide by 512. My approach is to use a 4060 as the reference oscillator and divide it down to 18kHz. I use a 4046 for the VCO and phase comparator, and a 40103 to divide the VCO down to 18kHz. The signal from the VCO is at twice the required frequency, so I feed it to a 4013 to divide by two. There are two complementary outputs at the output frequency, which I use to drive pairs of FETS through paralleled 4049 gates.

Modulating the PA is quite easy - there are some really cheap PWM amplifier modules from China. I use mains transformers as modulation transformers, usually as a tapped autotransformer. It's possible to build a 25W carrier, 100W peak rig for <£40. I've got a cheaper PWM - Based rig that I've been experimenting with, that delivers 40W carrier, 175W peak (it's LOUD). It costs about £30, but is harder to build.

I'll put some circuits up here!

[Gigahertz]

When I did some tests with mosquito transmitter awhile ago, the best success I've had was using a 10M roachpole with capacity hat and a loading coil at the bottom.

The loading coil consisted of a piece of waste pipe bought at DIY store, It had tapping points at certain number of turns. It was time consuming making loading coil but got me an acceptable match at these frequencies.

To have the ability to fine tune, build yourself a variometer https://wg2xka.wordpress.com/the-variometer/

You really need a antenna analyzer to assist you but if funds are short build a RF current meter and attach it to the wire to measure the current.

https://ac0c.com/main/page_homebrew_rf_ ... meter.html

[sinus trouble]

Interesting subject China Cat!

I have often had a similar thought of using a helical antenna for higher frequency applications!

My success rate with monopole antennas is pretty poor to say the least.

Thats not to dismiss it as a waste of time? But even if you get it to resonate at a reasonable match? You may have to accept that it will be a compromise between portability and effective gain?

[ChinaCat558]

Cheers for the replies everyone, much appreciated!

[Gigahertz]

Found this picture when I was testing a 10w AM transmitter awhile ago. Just holding a tube just near the antenna to make sure everything was working at a remote location.