Making a crazy dipole???

[Cagao]

Hi all,

I've got a RS Pro 2042 and currently a sky scan desk top discone antenna.

I'm interested in picking up the NOAA satellites but with the discone antenna I find myself prancing around the lounge with it in my hand trying to track it manually (not fun!).

Anyway, the other night I took the 12 radials off the discone and built a (double) dipole, to create a horizontal cross. That worked okay-ish (then realised i had my antenna cable laid over an 8 port mains adapter!)

Anyway, I'm now looking to build a decent dipole antenna for the 137MHz range.

I understand the half/quarter wavelength stuff, but struggling with some other area's of antenna design. Don't worry, I'm also working my way through thw Wiki. )

My questions are these...

If i make a cross shape with 2 dipoles, is there anything wrong with using more and building up a horizontal star shape? Will it hinder my receiving power in any way? I figure it would help as the satellite obviously changes it's polarization slightly as it flies overhead.

Also, what about having a dipole, with another smaller dipole above it, with another smaller one above that? Like TV aerials? Or would that make the whole antenna TOO directional?

I think that'll do for now, I'd appreciate as much guidance as possible, and I'm sure there'll be more questions and queries to follow. )

Thanks,

Rob

 

[KC8JPZ]

I do not fully understand your question. I do think you might look into another style of antenna. Here is a link to a site that will hopefully help you out.http://www.cebik.com/vhf/fsat.html

 

[Cagao]

Looks like some intensive reading! Thanks for the link, although I didn't think my question was too tricky.

 

[KC8JPZ]

Perhaps I've looked a little too far into the question. Check your private messages when you get a chance.

 

[Al42]

If i make a cross shape with 2 dipoles, is there anything wrong with using more and building up a horizontal star shape? Will it hinder my receiving power in any way? I figure it would help as the satellite obviously changes it's polarization slightly as it flies overhead.

A turnstile antenna (2 dipoles at right angles) is actually a sort of circularly polarized antenna, so it will receive the signal as the polarization changes.

Also, what about having a dipole, with another smaller dipole above it, with another smaller one above that? Like TV aerials? Or would that make the whole antenna TOO directional?

TV antennas are multi-channel - 1 element for the lowest channel, the next element for the next channel, etc. You're receiving a very narrow band of frequencies, so you don't need that.

Try some of these links.

 

[LarrySC]

The secret to any antenna is GAIN. If the sat is moving the antenna will have to move also if using a diapole. You can get a VHF 360deg 5/8wave 3db gain MAXRAD and mount on a NMO base adp. You can google ARRL and look thru Sat antenna ideas. Good Luck

 

[Cagao]

Thanks for the info, I thought the idea of smaller and smaller dipoles was to increase the gain for the one at the 'back'?

I see what you're saying with the crossed dipole, but wouldn't there be a weakness in each quadrant? Or is that pretty much non-existant?

Thanks for the link too.

 

[KC8JPZ]

Absolutely there is a weakness in the quadrant. Not typicaly allot however. To pickup that missing quadrant would require a helix or helical antenna. Looks like spring. http://www.qedata.se/bilder/projekt/helix.jpg

There is a difference between right and left hand winding on these antennas .

 

[Cagao]

I've just thought about this, which is probably rediculous, but i'd be interested to know if or why it wouldn't be a good idea...

A turnstile dipole, but with another element pointing up, and another pointing down, so in effect, it wouldn't matter if it was placed horizontally or vertically, because you'd still end up with the same model.

What would happen to the lobes and nulls of the plain turnstile dipole with this arrangement? Anything? or would it simply give you the extra polarization to pick up signals?

Cheers,

Rob

 

[Al42]

A turnstile dipole, but with another element pointing up, and another pointing down, so in effect, it wouldn't matter if it was placed horizontally or vertically, because you'd still end up with the same model.

Look at a plot for a dipole. It receives just as well up, down, sideways - just not off the ends. But the other one will receive off the ends of the first one.

Try a turnstile - you can make one (it won't hold up for too many winters, but it's a "proof-of-concept" antenna) for a few dollars. A roll of wire (preferably copperweld if you can still find any) and some scraps of lucite or other plastic. If it works (it should) you can pretty it up later.

Larry, for WeFax the secret ISN'T gain - most receivers can receive WeFax just fine with a dipole - the answer is polarization and view angle. Since a turnstile covers the entire sky the antenna doesn't have to move. But your Maxrad does - the satellite is constantly rotating, so the polarization is constantly rotating too. When the satellite's polarization and the antenna's polarization are crossed, there's about 20db polarization loss. Not good. So look at the ARRL site, Larry - for WeFax antennas. You'll see a lot of turnstiles.

 

[Cagao]

I've already made my turnstile, 4 copper pipes, and appears to be doing the trick as well as can be expected.

The reason i queried the extra 2 elements was thinking about airband signals that are vertically polarized (since they'd need approximately the same length of copper) - if they are anyway???

Also, can someone explain the connection between the two dipoles a little better for me? Currently I've simply got a dipole, then another element attached to one, and another to the other, if that makes sense. I see illustrations showing a 90degree bit of wire sometimes, other times a balun.

Bit of explanation would be great, i'd prefer knowing more about the simple bit of what looks like coax drooping down to connect the extra dipole.

Thanks in advance.

Rob

 

[nd5y]

The coax loop is a 1/4 wave transmission line section that feeds the two dipoles 90 degrees out of phase and creates circular polarization.
If you don't have that then you don't have a turnstile. If you just connected the dipoles directly together
you just have a bow tie antenna (wideband dipole) and not a circular polarized turnstile.

 

[Cagao]

Does it HAVE to loop downwards, or is that just how it's shwon in diagrams?

 

[KC8JPZ]

The phasing harness is not affected by direction. It can be any direction as long as you have the correct length.

 

[Cagao]

So this looks okay?

http://rob.intelcompute.com/dipole/1.jpg
http://rob.intelcompute.com/dipole/2.jpg

The black coax is 75ohm and 1/4 wavelength just like each 'element'.

 

[KC8JPZ]

Looks good from my house. The desk mount might look a bit funny on a tower.You might want to hit the connections with some solder. Other than that. Hows it work?

 

[Cagao]

Not too bad...

http://rob.intelcompute.com/dipole/satellite.jpg

I'm in the middle of the UK btw.

Although I did find myself having to direct it a bit to get a perfect signal. :-/

I'm on the 2nd floor of a 3 story block of flats, so that's probably killing it slightly, plus i'm using a Radioshack Pro 2042 - looking for a better scanner on eBay now with better IF filters.

I was reading a review of a scanner and read this...

The receive circuitry is based around a triple superheterodyne with I.F.s' of 622MHz, 10.7MHz & 455kHz. Multiple switchable I.F. bandwidths are available in both the 10.7MHz and 455kHz I.F. stages: 3kHz, 6kHz, 15kHz, 40kHz, 110kHz & 220kHz with provision for a 500Hz option.

What's the 10.7MHz and 455MHz all about? From what I've read, I want an IF filter of around 50-60kHz, which this one sounds like it does with the kHz options listed, but what's the MHz frequencies all about?

Cheers,

Rob

 

[Al42]

The 137 MHz signal is mixed with a 485 (approximately - depends on 137.<what>) MHz local signal (this is the one that changes as you tune to different frequencies) to produce a first IF of 622 MHz. (This is basically so that you can't receive an image of 800 MHz cell phone frequencies in the US, but it turned out that "designing to the law" produced a better receiver concept.) This signal is mixed with a 611.3 MHz signal to produce a 10.7 MHz IF. (That's a bit sloppy, since conversions should be held to 10:1 at most, to avoid image problems, so the second IF should be no lower than 60 MHz, meaning that a receiver with a 455 KHz last IF would need 4 conversions, not 3.) The 455 KHz IF is there for selectivity - the lower the frequency the narrower the bandwidth possible.

The question (I'm not that familiar with signal structure for WeFax) is whether you want an IF at 50-60 KHz, or you want an IF with a bandwidth of 50-60 KHz. An IF of 50 KHz is going to be pretty narrow - too narrow for voice and definitely too narrow for 15 KHz FM signals. If WeFax requires an IF bandwidth of 50-60 KHz, the specs claim to be capable of that - either 40 KHz or 110 KHz would do. (Too narrow would "fuzz" the display a bit. Too wide would allow adjacent signals to get through - if there are any - and would raise the apparent noise level just a bit.)

If the receiver you're looking at is reasonably priced, and you're serious about satellite Fax, I'd go for it. It certainly has things the Pro-2042 never thought of. The only question I'd have is the sensitivity at 137 MHz. If it's no better than the 2042, you won't get a much better signal.

 

[Cagao]

Can I have all that again please, in English. )

Well and truely over my head I'm afraid. You've mixed with a 485MHz signal, then a 611.3MHz and end up with a 10.7 MHz IF?

I (rather stupidly it seems) thought the IF was just how much the scanner will deviate from the frequency you're tuned into?