Antenna design

[DBM]

if you've ever built a germanium diode radio you know that you need a very long aerial stretched high between masts and a solid earth ground in order to get the best reception. however this is impractical for portability. i am wondering if it is possible to fashion an antenna somehow compact perhaps coiled and some kind of derived ground plane in order to make the receiver portable and approaching the level of performance of a long aerial and solid earth ground. all antenna design i can find deals a lot with tuning. but what if i want broad band reception in case i want an RF detector and not just AM band. what im thinking is a short antenna on one side of a coil and the coil itself wound in a way that it serves as a ground plane perpendicular to the antenna. any input from someone more educated than me in antenna design would be appreciated.

 

[davenn]

For a crystal set type radio, no not really. Specially if you are any distance from the transmitter. Because there are no amplification stages in the radio it totally relies on the strength of the received signal and that is reliant on the amount of wire in the air

No the coil can't/ won't act like a ground plane it will act as part of the tuned circuit.
With a short whip, the unit will only act as a RF detector over a short distance anyway, a few metres or so at the most from a transmitter.

Dave

 

[(*steve*)]

You could investigate a loop antenna.

It is directional, so it *may* assist you also in cases where your radio keeps picking up a strong local station.

 

[DBM]

thanx for the input. I will boost the signal electronically. no problem there.

My circuit will control another circuit in the presense of any RF. thats why im not concerned with a tuned antenna.

as far as my coil is concerned, i believe the best design is a dual coil design. an antenna connected to a 18- 26 wraps coil wound over a second winding with more wraps. the signal recieved by the antenna falls across the short coil is seen by the second long coil and is increased by the winding like a step up transformer. then the germanium diode detector will see a larger signal which i can boost from there for my needs. i believe if i connect the two sides of the coil opposite of the antenna will give me somewhat of a derived ground point to work with.

i can handle the electronic design, just not the antenna. all i need is an compact antenna to preform like a long aerial. im concerned that a teloscoping radio antenna will not recieve anough signal.

 

[DBM]

i'm tempted to try using a CRT degaussing coil connected on one side in place of an antenna. not sure how long that is but it might be able to boost a signal passing through multiple wingdings...

 

[duke37]

To capture a lot of energy you need a big antenna and a minimum amount of wire to keep resistance down. There is obviously a compromise here. Any transformer will absorb energy.

I suggest you look up 'pi match' to see how an antenna can be efficiently coupled to your receiver.

There is a compromise between efficiency and selectivity, controlled by the 'Q' of the circuit.

A loop antenna couples in to the magnetic field and this can be enhanced by including a low loss magnetic rod - hence the ferrite 'loop stick'.

 

[DBM]

the best way to keep down resistance is to shorten the length of the wire antenna or use a larger gauge diameter. multiple short pieces will give me the length as well with less resistance but now im dealing with tuneing the antenna again same as with Q. Ive looked at several designs. loop. coil, helix, cone... what ive found so far for bandwidth is a dual spiral cone shape. something about the shape makes it usefull for recieving radio wave across multiple bands.im thinking solid #10 awg copper or something larger like copper tubing supported on a pvc mast.

 

[BobK]

The resistance in an antenna for a receiver is not likely to be a problem.

Bob

 

[DBM]

i come across a "Tesla's ghosts radio" which used a football shaped antenna that was quite powerful and is inverted in shape from the dual cone coil i was looking at. In fact the whole shebang is almost exact to what i have minus the tuning cap. the wide ends are together not the narrow ends. I may take his designs into consideration. certainly worth experimenting with.

 

[DBM]

The resistance in an antenna for a receiver is not likely to be a problem.

Bob

i agree. i think no matter how much resistance is in an antenna as long as it is a open loop then i can look at it as if it is a high impedance input. in other words, there is so much resistance seen at the input that no load falls across it.

 

[duke37]

I dissagree.
The resistance of an antenna is not a problem for most receivers since the set has gain to make up the losses. In the case of the crystal set, the output power all comes from the antenna and so losses are catastrophic. A long wire will have low losses but a loop will have high losses due to the many turns.

Also, it is important that the antenna is power matched to the set to get maximum power transfer, this should be done with the lowest loss circuit which means low Q so poor selectivity.

Diddy antennas will be of no use for a crystal set however they are configured.

 

[DBM]

i dont remember the resistance per foot of copper off the top of my head and looping the wire will add inductive reactance which only affects AC. there is no way out of these but i do know that energy passing through winding is going to be more received than energy passing through a single strand and even more so if it is wound bifilar and that the XL will be a small value