helix antenna question

[Muckluck]

I recently purchased an Icom IC-R3. I like the radio, but I have read that it is a little weak when scanning in the 1GHz to 2.5Ghz (FM Video) range. I have also read a few websites about how to build a helix and it looks fairly easy. My question is... is a helix only useful for "site to site" matching helix TX/RX?

In other words, would a helix do me any good for general directional scanning? I realize that even a "bad antenna" will improve reception, but can I expect significant gain in the frequency that the helix is tuned for?

TIA
Muckluck

 

[prcguy]

A helix is probably not regarded as a general purpose scanner antenna. A properly designed helix has about 30% bandwidth, in other words if you make one centered at 1GHz, it should be useable from about 850 to 1150MHz, etc. A helix is circular polarized and you will loose about 3dB os signal when pointing at a linear polarized antenna, which is most all communication antennas. This means if you make a 10 turn helix, which has around 10dBC gain (I forget exactly) it will behave like a 7dBD gain antenna when pointing at most other antennas out there unless they are the opposite circular pattern. They also have about 140 ohm impedance and there are several ways to match a single helix to 50 ohms. The most common way is to have about 1/4th of the first turn be very close to the reflector (about 1/8”) and usually you need to solder a thin strip of sheet metal to this first ¼ turn to increase the capacitance to ground. The drawback of this matching is it narrows the bandwidth some. You would benefit from some test equipment or at least being able to transmit and measure SWR when doing all this. Some of the Radio Amateur Handbooks have this info and a guy I worked with wrote the article on this specific matching technique. One of the best hobby uses for a helix is UHF satcom, where the bandwidth and circular polarity are just right for the 245 to 275MHz range. I did make a 5 turn helix years ago for a project and it was centered in the 460MHz range. It was kind of cool to rotate it and not have signals fade out due to cross polarization nulls.
prcguy

 

[Muckluck]

thanks for the info. Thats sort of what I figured based on what I read. I may go ahead and build a couple anyway and play with them on my 802.11 network...

 

[kb2vxa]

Hi Muck and all technically inclined,

You might also try your hand at a loop Yagi. I don't remember exactly where I saw one made from a Pringle's can, could be www.southgatearc.org is a good place to start. I have seen them around so there must be something to them.

For prcguy;
"I did make a 5 turn helix years ago for a project and it was centered in the 460MHz range. It was kind of cool to rotate it and not have signals fade out due to cross polarization nulls."

Now you see why they're used for satellite work where the polarization changes constantly. They have been largely replaced by cross Yagis which can also be set up for circular polarization. They're good for terrestrial work such as FM/SSB operation with a single transmission line, you don't need a second and a switch. With such high gain a 3dB loss is quite acceptable. BTW, the last time I saw a helical array was in one of those 50s sci-fi movies tracking a V-2, I think it was The Three Stooges Go To Mars. (;->)

 

[Muckluck]

Yes, am quite familiar with the "cantenna" for 802.11. I have built several to try different things. They work well for bluetooth frequencies also. My best working one is a Pirouette (SP?) chocolate filled cookie can with a 4" to 6" aluminum heating duct adapter soldered on. Works great!

Since picking up the scanning hobby, I thought I would try to apply what already know, add some additional knowlege and build something new (at least new to me). The cross yagis sound interesting also (a ham that I work with mentioned it recently).

Thanks for the input...

 

[mancow]

Did you check out my pics in the space monitoring forum?

Also, a slinky jr is the exact size needed for 2.4 Ghz.