RF antennas

[Asjad]

Dear All,

I am trying to build an aerial for a walkie talkie, since the built in antenna
is rather pointless.

I came across the the 1/2 wave dipole.

###I understand the principle that each 1/2 of the dipole represents
1/4 of the target frequency ###


When you have made one, you have 2 wires to connect to the walkie talkie.

(1) Am I supossed to connnect one wire to the aerial of the walkie talkie
the other lead (coax screen) to ground????

(2) Will this type of aerial transmitt and recieve radio signals???

(3) Is this aerial directional transmission, if not which type is????
-----------------------------------------------------------------------------------

(5) At 50Mhz the aerial would be 3 meteres, is there any techniques I can use to reduce the size, possible keywords to search textboooks/google???

YOUR REPLIES ARE GRATEFULLY WISHED

 

[JimB]

Asjad

I dont undersatnd your comment that "the built in antenna is rather pointless". What do you mean by pointless?


The dipole.
You are correct, each leg of the dipole is 1/4 wavelength long at the operating frequency.

If you connect the dipole to a length of coax cable, the centre of the coax will then connect to the "antenna terminal" of your radio,and the screen of the coax will connect to "chassis" of the radio, probably the 0v earthplane on the circuit board.

Yes this antenna will transmit and receive well.

A dipole is directional, you will get very little signals of the ends of the dipole and strong signals off the sides (at right angles to the run of the wire).

Yes a 50Mhz dipole will be about 3 metres long, it can be made shorter by inserting a coil into each leg of the dipole, or, by winding the whole of each leg into the form of a coil.
When you start doing tricks like this, you are into a whole new area and need some test equipment to adjust the antenna for good performance.
By the way, a shorthened dipole will not work as well as a full size one.

There are numerous books on antennas published by the RSGB (www.rsgb.org) and the ARRL (www.arrl.org).

JimB

 

[Dean Huster]

"I am trying to build an aerial for a walkie talkie, since the built in antenna
is rather pointless."

I could expand this into a pun for you (Ron H. would like that) and ask, "Do you mean that the antenna is non-directional/omnidirectional?"

Unless the walkie talkie has provisions for an external antenna (a provided jack or the "internal" antenna is removable), it's highly unlikely that you'll be able to construct and antenna, coax system and proper connector that will work as well or better in the gain and SWR categories than the manufacturer's original antenna. By the way, that original antenna, if designed and made correctly, should be a resonant antenna for the frequency in use, so it isn't like the dipole will impart any new abilities (other than a bi-directional signal pattern) to your transceiver.

Dean

 

[Klaus]

Also, keep in mind that a dipole is a balanced antenna, it should have a balun when connected to an unbalanced output like your inbuilt antenna.
The balun also can correct the impedance mismatch between those different antenna designs.
Best you read up on the books suggested above, antenna's are a rather complex subject.
Klaus

 

[pike]

Excuse my ignorance but when you say half-wave antenna do you mean it in terms of wavelength???

I read this principle somewhere but didn't get it. This was what i was told:

"The higher the frequency the shorter the antenna can be because it has a smaller wavelength. So if electricity/light travels at 300 000 Kilometres, @ 100 mhz the best antenna length should = 300 000/100 000 000hertz

Therefore a 0.003 km or a 3 metre antenna would be optimum"

Is this really true???

 

[stevez]

The intent of an antenna is to radiate the power from a transmitter that is delivered by the transmission line. One simple but effective antenna is called a dipole - essentially two wires (or other straight conductors) that are connected to the feedpoint and stretched tight to be in a relatively straight line. How the antenna behaves in terms of radiation pattern and what it looks like in terms of electrical load is determined in part by it's length. A common arrangement is a half-wave dipole which is as you might guess - a dipole with a length that is approximately 1/2 wavelength at the frequency of interest. A common formula to get the length of a half wave dipole - 468 divided by the frequency in mHz will give the length in feet. This is somewhat different than the pure 1/2 wavelength and accounts for some other things.

Height above ground and proximity to other conductive objects (that include the user) impact the behavior of an antenna to a greater or lesser degree. The output circuitry of the transmitter, for a mobile or portable application must deal with an ever changing antenna and does so by cutting back on the power - or in some cases, by retuning the antenna.

There is so much more and fortunately most of it in publications like ARRL handbooks or similar publications by RSGB.

 

[zachtheterrible]

When calculating how long 2 make ur antenna, the 1/2, 1/4, etc. mean that each side of the dipole is 1/4 or 1/2 wavelength long. Your whole dipole is not 1/4 or 1/2. I think that's true :lol: .

One question ive got about dipoles is if you take speaker wire and pull it apart till the desired length is reached, is it possible to have 2 or 3 feet of wire that is not pulled apart, because wouldnt this act as a ribbon cable?

 

[stevez]

A dipole antenna is usually 1/2 wavelength or an odd multiple of 1/2 wavelength. Each 'half' or side of the dipole is 1/4 wavelength long.

Speaker wire (paired conductors) could be considered a balanced transmission line. You would have to measure or calculate the characteristic impedance of this line. If you pull some of it apart and arrange it at 90 degrees to the paired conductors then the paired conductors could be considered feedline or transmission line. Ideally the dipole (wires that are pulled apart) would be resonant at the frequency of interest. If the impedance at the feedpoint, where the wires come together, is the same as the characteristic impedance of the paired conductors then there will be efficient transfer of power from the feedline (paired conductors) to the dipole antenna. The line that connects to the end of the paired conductors requires a similar match for efficient transfer of power.

A friend of mine measured some common 'paired conductors' with TDR and discovered that the characteristic impedance of things like Romex (common house wiring) was around 130 ohms. If you know the insulating material of your speaker wire, the wire diameters and separation you can calculate the expected characteristic impedance with some pretty easy math.

The actual feedpoint impedance of a dipole is dependent on a number of things. Height above ground and proximity to other objects has a profound effect.

Dipoles are somewhat directional but only if sufficiently above ground and away from things that might influence the radiation pattern. An example from memory - a dipole for 3.75 mHz that is 20 ft above the ground will likely have a radiation pattern that isn't very directional or desireable (straight up). Note that a dipole for 150 mHz at 20 ft would behave more directionally because it is several wavelength above the ground.

While perfect matching between antenna and transmission line is desireable there is actually quite a bit of tolerance in terms of how much mismatch it takes to result in significant degradation. Many good antenna builders provide some method for adjusting the feedpoint so that the antenna matches the transmission line. A gamma or 'T' match are examples.

What many radio amateurs do is use the formulas, modelling and other design tools to do is come up with a place to begin. Then with the aid of antenna meters, dip meters, SWR meters, in-line wattmeters, etc they test and measure the actual perfomance then make the necessary adjustments for proper performance.

 

[zachtheterrible]

wow steve, that explained A LOT!! it really helped. thank you

btw, does anyone know of any places on the net where i could read up on how measure impedance, do impedance mataching, etc? eventually im going to get those AARL and RSGB handbooks, but until then . . .