Questions about Antenna...

Fischer777

For a project I was going to build a simple 160MHz transmitter/receiver using a Yagi-Uda antenna arrangement and a resonant circuit. In some diagrams I notice that there appear to be two antenna outputs/inputs. Are these outputs spliced somewhere before reaching the LC circuit, or is one end grounded and the other attached to a part of the resonant circuit?

vk6kro

Do you mean the two sides of a dipole as two inputs, or two inputs on a larger antenna?

A dipole is like this:


That thing they call a "choc block" is just a way of joining the wires together.

Fischer777

Something like this picture here: http://personal.ee.surrey.ac.uk/Pers...ded-dipole.gif It seems there are two outputs, and I don't know if they are eventually spliced together before attaching to the resonant circuit or if they both remain separated.

skeptic2

Is this a project for school or for your job?
How do you intend to use the transmitter/receiver?
How much range are you trying to achieve?
How did you decide to use 160 MHz?

vk6kro

The two wire ends are regarded as one input or one output.

A folded dipole has an impedance of about 300 ohms, so you might use 300 ohm TV ribbon to connect to it, but it is similar to the coaxial connection above.

Either way, the two wires of the transmission line connect to the two connections on the antenna.


When it gets to the first tuned circuit, it might connect to a loop of a few turns of wire wrapped around the coil of the tuned circuit.

Fischer777

It's more of a hobby project that anything else.

I am using it to see if I can transmit energy in the form of radio waves. Given that, I don't expect the range to be more than a few inches, possibly a foot or so.

I'm not dead-set on 160 MHz, it's just what I calculated it's frequency would be with a 6-turn 10mm-in-radius inductor on a 6 pF capacitor.

So here are three interpretations of how this antenna is hooked up to a resonant circuit.

I'm not sure which one is the 'correct' one, but that is essentially what I'm asking. Which of these connection schemes will work best (I am assuming that for the ones where there are two distinct outputs it's being run through a co-ax wire)?

vk6kro

The best answer is "none of the above".

A and C have no chance, because only one of the wires goes to the tuned circuit.

B is closest, but it should not go across all of the coil.
There should be only a few turns of wire between the two wires of the coax.
You could ground the bottom end of the coil and connect the shield of the coax there.
Then connect the centre wire of the coax about 20% of the total length the coil from there.

If you do use coax, it would be better to use a simple dipole rather than a folded dipole. This is because coax has an impedance of 50 or 75 ohms and this is closest to the impedance of a dipole which is 72 ohms.
A folded dipole needs 300 ohms.

Post No 2 shows a simple dipole.

Fischer777

So is this what you mean by only a few turns of wire between the two coax terminals?



Is there a formula to calculate the optimum spacing between the two turns of wire? The resonant circuit I'm using will at most have 6 turns of wire in the inductor.

vk6kro

Yes, that would probably work. You might find that the coax needs to be across only one turn of the coil. Or, you could make a single turn and place this around the main coil. One of the diagrams in this thread shows how to do this.

The tapped coil is just like a step-up transformer. Its main function is to avoid loading the tuned circuit too much. If it was loaded too much, then it would tune very poorly.

There are formulae for such things, but it is easier to just try it and see.

You haven't said what you want to use as a transmitter. What is your signal source for this experiment and what do you intend to use as a receiver?

Fischer777

I read that the physics behind transmission and reception in antenna were essentially the same, and could be used interchangeably. I figured I could use the same design for both the transmitter and receiver.

The transmitter is nothing more than a modified FM frequency jammer (like here: http://www.eleccircuit.com/wp-conten...ing-2n2222.jpg ) that is fixed at a certain frequency. I will try and focus the radio waves using an antenna, and project them to a receiving antenna (as I said, I think I can use identical antennas for this). The receiving antenna will be attached to nothing more than a bare LC circuit (same as the one in the transmitter) with a load (such as a low-powered LED). The purpose is just to demonstrate the transmission of wireless energy using radio waves.

berkeman

And how many licensed receivers do you suppose you will interfere with while you are operating your unlicensed transmitter? Do you even know what bands are near the 160MHz target operating frequency of your transmitter?

Fischer777

I live in a fairly isolated area out in the country surrounded by open fields. Given the distance between my house and the next one over, that I'm using a relatively weak transmitter, and that I intend to narrow the frequency beam so the majority will be concentrated at the receiver, interference should be minimal.

Averagesupernova

If you had actually chased down signals that were interfering with your own receiver you would be more concerned than you appear.

Antiphon

The band around 160-162 is used by railroads. If there are tracks within 10 miles of you, you should consider a different frequency.

Fischer777

I'm not sure what you mean by this. I understand that there can be severe legal consequences of jamming signals by use of unlicensed transmitters whether intentional or unintentional.

There are, but I'm not set entirely on a 160 MHz frequency. In fact, I'd like to get it in the 300 MHz range, so I can use a smaller antenna. Once I figure out what frequency I'm going to use I'll figure out what it could possibly interfere with. I do have a metal shed (walls, ceiling, doors, everything made of metal), so I'm thinking if I test it in this it will act as a RF Faraday cage.

Averagesupernova

What I mean is that when someone from a two-way radio company or similar business spends time and money searching you out what will you say when they find you? Tracking down interference is time, money and resources spent. Will you offer to compensate them? Not likely. Of course there is the possibility they will NOT find you before your interference goes away and have no way to recoup the expense at all.

berkeman

The problem is that in order for you to detect the received signal, you will need a very sensitive receive circuit, or you will need to transmit enough power so that a simpler receiver can detect the signal.

It does not sound like you have looked into what is involved in building a sensitive receiver yet. I think that might be a better project for you at the moment, rather than experimenting with transmitters in licensed bands.

You could build a broadcast frequency receiver for the FM band, for example. That is a useful project, and will teach you a lot.

You can also look into amateur radio (HAM radio). You don't need to learn Morse code anymore in order to get your license, and once you get your basic technician HAM radio license, you are allowed to transmit on a number of bands (including some around 155MHz). You can use the hobby books to learn how to build transmitters that comply with the law and do not interfere with other licensed bands.