OK, I know this is probably asking way too much but I want to direct an HF signal (say 27MHz initially) into the ground from a portable antenna. A lot of Googling has turned up all the usual antenna types but I was wondering if there was something that could be done to produce something more uni-directional.

The reasons for wanting high directionality is to get as much energy as possible into the ground, reduce noise and interference and as far as possible to ensure the receiver is receiving a signal from the ground rather than via other paths.

So suggestions please? Ideas I've had are an active antenna inside a metal tube, a loop antenna with a dish or plate on the back of it, some kind of loop with parasitic elements. I don't know a great deal about antenna design and don't know where to start with modelling these things, is there free software that would help?

 

The wavelength of 27Mhz is about 10m. A highly directional antenna will have dimensions many times this length.

Also the "beam" will not be correctly formed until many wavelengths away from the antenna.

 

Thanks but I am not looking for a classic directional antenna, I don't particularly care about a narrow beam once the signal is in the ground. Perhaps it is better explained as wanting a highly uni-directional antenna where there are no or very small side or rear lobes. I accept this might not be possible but I have seen small loops work quite well so a small loop with some kind of rear lobe suppression seems like an option for experimentation.

 

sorry, but lobe supression and directivity mean large size. and an active antenna, even inside a metal tube would be for recieving, not sending energy. a small loop has a broader pattern than a larger loop. some claims have been made for fractal antennas, but I have not seen any working versions for hf. the smallent directive antenna for 27 mhx would be a half wave square loop, about 4.5 ft per side.

 

And loops are bidirectional along the loop axis...

 

Here is the kind of thing that Google turns up - http://www.aa5tb.com/loop.html

I am aware that this kind of antenna will be bidirectional but what I was pondering was something like one of those in a tinfoil tent?

 

The Antenna Modeling software you want is called EZNEC. It is free.

When you say you want to direct an HF signal into the ground I think your frequency range is too high for any meaningful ground wave propagation. Going lower in frequency increases the chances of ground wave propagation but increases the size of the required antenna and reduces the directionality. I think your quest is nothing more than a vain hope.

http://en.wikipedia.org/wiki/Ground_wave_propagation

 

Thanks for the link regarding the software. I am aware of the the work that has been on on cave radios for example but we have picked up amateur transmissions from within a cave on 10MHz through tens of metres of rock on a loop antenna so it may not be as vain as you think. One question that needs answering though is what is the propagation path.

 

Receiving is one thing and transmitting is something else again. We started at 27 MHz and now we're down to 10 MHz. One third the frequency requires antennas about three times larger. The longer the wavelength the harder it is to make a directional antenna.

 

the propagation path is through the earth for a cave radio.

 

Papabravo - I was giving the 10MHz example because that is the example I have. I still intend to try 27MHz just haven't done it yet.

Alfacliff - well yes but are the preferential paths?

Anyway this is all getting a bit off topic, I wasn't really looking to debate methodology, just looking for advice on the OP and the answer seems to be that it can't be (or hasn't been) done. Thanks anyway for the input.