h field simulator antenna design help 1

[jgameros]

Hello,
I am designing a Loran simulation tool that uses h-field propagation. I have not been able to find information on how to design the emitters I need. This is a three emitter system that need only transmit 18 to 24 inches. My available output is 1Vpp currently. Does anyone have suggestions on where to start? Thank you.

 

[VEBill]

You're not using Loran-C's 100 kHz frequency band, are you?

Over 18 to 24 inches?

 

[jgameros]

Good morning,
Yes I am simulating Loran C. The transmission distance is small as I am attempting to radiate a Loran chain (inside a Faraday cage) in order to test a Loran receiver/antenna system.

 

[VEBill]

What sort of antenna is under test?

If it is a whip, then you could just use a small value (pF) capacitor and clip it onto the antenna. Feed the signal(s) into the other side of the capacitor.

If it is a loop, then you could use a similar loop and essentially form a transformer.

It might be worth adding a variable attenuator inline so you could compare MDS.

This type of approach sort-of bypasses actually testing the antenna itself (as an antenna). But they could be qualified (once) outside using the actual Loran signals.

I was initially confused by the stated range ("18 to 24 inches"). I thought that maybe you wanted to move the UUT around and see an effect (you wouldn't). I'm now assuming that a direct connection (zero inches) is acceptable.

 

[jgameros]

This might help explain what I am doing, the attached photo is from Loran.org's website. It shows a UUT centered between three antennas. I am trying to create a similar type of bench however I do not have much power to work with (they used an actual transmitter).

 

[VEBill]

Those three black rods look like ferrite rods of the sort found in old portable AM radios. If so, then they're simply loops (typically 50 turns or so).

I thought that Loran was a time-of-arrival system (not depending on direction of arrival). I recall seeing Loran receivers with whip antennas (non-directional). So I'm wondering why the test set-up is arranged with the three directions spaced at 120 degrees.

Off I go to investigate...

 

[VEBill]

"Added feature... The eLoran Heading output using Loran-C provides bearing accuracy better than 1º, making the e-LORAN a practical alternative to more expensive heading devices."

http://www.si-tex.com/html/e-loran.html

 

[jgameros]

The three antennas are indeed iron core. The reason for the approx 120 DEG alignments are position of the antenna and as well as time of transmission determines heading.

 

[VEBill]

They're also doing DF-ing on the Loran signals. That's something 'new' (to me).

A simpler approach to testing such a rig would be to have one source that runs around on a track circling the UUT.

Or just go outside.

 

[jgameros]

The reason why I said approx 120 DEG is because they are not truly 120 DEG. They are positioned relative to true north as well as the simulated location of the receiver to the Master, S1, and S2 transmitters. It is line of bearing to the real world transmitter stations in a particular chain.

 

[jgameros]

The simpler approach you suggested is exactly what I am attempting. The issue I am trying to address is what I would need to use for an antenna. since the receiver I am testing is using an H-field, I believe I need additional power than my current source in order to create a strong enough field to be received cleanly. Is this not the case?

 

[VEBill]

When I were a wee lad, I built a metal detector that used a portable AM radio as the receiver. The signal from the detecting coil had to travel about two feet. The oscillator was powered by a 9-v battery. The signal was very strong.

So, assuming that you have a free hand to use whatever coil is required, and can place it closer if required, then my gut tells me that you've got a fair shot at it working.

But maybe you should do some quick and dirty experiments before investing too much.

There will be an optimum coil design. Too much inductance and you'll have reduced current. Too few turns and you'll not have enough amp-turns.

 

[jgameros]

Thanks for the information. I am going to set up everything I have already and see if I can get an optimal antenna into the bench. It seems as though I should be able to generate the Loran signals and have them picked up within a bench that is relatively small...say 24"X24" square. Would you agree?

 

[VEBill]

Assuming that the noise floor is low (inside a shielded room). Assuming that the transmitting 'antenna' is reasonably close to optimal. And assuming that you're lucky...

 

[jgameros]

The noise should be well reduced as this bench will be a Faraday cage (not 100% isolated but very close). And I would say that I am a pretty lucky person, case in point I found this website.

The optimization of the transmitting antenna is my only major hurdle to getting this bench to work. Trial and error is what I am inferring from your string. Do you have a suggestion on a starting point? I was thinking of taking the receiver antenna apart and using its coils. But I think the inductance will be too great to be effective as a transmitter.

 

[VEBill]

Assuming your H-field antenna-under-test is based on loops; are you allowed to treat it as a transformer and place your source loops right on top for maximum coupling (if required)? A 'transformer' with 1V in and requiring only about 1uV out is pretty uncritical.

For a ferrite core, maybe start with about 50 turns. For air core, maybe three-inch diameter and 50 turns. These are pure guesses.


If you're trying to qualify a new antenna design, then it'd be far easier to step outside. Otherwise you'd be heading in the direction of an RF goniometer (three dimensional coil arrangement). And then the available 1V will be the least of your worries.

 

[jgameros]

Excellent, Thank you so much for this information...it is a great place to start. I will let you know how it goes.

Thank you again!!