Anyone know about electronics?

[dave]

Does anyone know anything about electronics/electrics?

 

I've posted on a few electrical forums but as such no answers.

 

I am in the process of making a quadrifilar helicoidal antenna (try saying that after a few drinks!) and have found a few designs on the good old Interweb, but out of the designs I have found where they make the antenna from coax cable, they use four lengths of coax cable and solder them together at the top and bottom.

 

My question is, does this matter? Can I just use one long length of coax to save the soldering?

 

Here is what I am making:

 

http://www.cidb.co.uk/pics/helicoid.jpg

 

...as you can see, it would be possible to feed one long length of coax through from the bottom, up around etc... and back down to the bottom to the connector and still keep the helicoidal shape.

 

One other thing is, I believe that the coax in the UK is 75 ohms - I'm not sure what the receiver that this is going to be plugged into is. Would I need to convert it (by maybe soldering to a heavier ohm coax cable) to the same before attaching to the unit or would I need a powered resistor of some sort, and if so, where can I get one and what would I need?

 

I was going to make it from copper tube/pipe, but I figured that there would be no impedence and I would probably short the unit the moment I connected it to it.

 

The unit that this is going into is an Icom PCR1000 (in case that means anything to anyone!).

Edited July 18, 2008 by dave

[daviet1976]

Have just emailed my dad

 

He's into this sort of stuff

 

He's actually a metrologist, but was formerlly an electrical engineer.

 

Will see what he says and get back to you....

[Guest wino]

He's actually a metrologist, quote]

 

What??? he worked on the underground?

[Rooster-UK]

Dave, need more info on the construction before comment.

 

Are the inner and outer conductors crossed over at the top and bottom? Etc.

 

Using it to drive an Icom PCR1000, which is a scanner, the difference between 75ohm and 50ohm cable wouldn't be too important. If it were being used as an aerial for a transmitter, then the difference would be significant as the mis-match would increase the S.W.R. and you would lose output.

It IS possible to get 50ohm co-ax cable in the UK, but it is not widely stocked so you would probably have to buy a 100 metre drum.... Expensive!

[dave]

Excellent info.

 

I've actually made it now (and quite proud of it too ;-) )- I have split the cables, but the impedence still bothers me.

 

However, if and when the Icom finally turns up (next day delivery last Monday!!!! Grrr!) I shall just plug it in and hope for the best.

[HSBCrusher]

anorak alert anorak alert!

[Lula]

anyone know wtf he is talking about?

[Rooster-UK]

Yes.

I used to be a TV/Video engineer. Also a licensed radio amateur.

For last 12 years working in industrial electronics.

 

I know what he's on about.

[honey5]

unfortunately i do too

[Rooster-UK]

Why unfortunately?

[honey5]

ok then not unfortunately, but talked too him about it at length

 

shall i get my anorak and leave ~~~~~~~~~~~~~>

[barracad]

Also a licensed radio amateur.

 

You need a licence to be amateur?

[saintly_1]

anyone know wtf he is talking about?

pass

[dave]

You need a licence to be amateur?

 

Yes, and that's when you get your call-sign.

 

I don't need one as I'm only receiving not transmitting.

 

Here is the (half-finished) product:

 

 

The bit that is now confusing me is this:

 

 

The bit in the top right where it says to solder the braid and the conductor from B to A. Does that mean that I wrap the braid around the conductor of B and solder them together and then solder both to the conductor of A, and then solder both from B to the conductor and braid of D, or does it mean to solder the braid to the braid and the conductor to the conductor?

 

The bottom diagram, even I understand ;-)

Edited July 20, 2008 by dave

[dave]

Just another quick question about this:

 

At the top of the house, I can visibly see the horizon from any direction; as the satelittes are polar-orbit, this should help I think. However, does the antenna have to be outside (not a problem, as I have a 'very' long wooden pole that I can raise the antenna on), or can I just hang it in the center of the house on the top floor - this would be easier, but can I expect the signal to be degraded because it's inside?

 

Sorry to bleat on about it, but I've tried posting in about 3 or 4 electronics forums and I haven't had a single reply, so I thought I'd try here.

Edited July 20, 2008 by dave

[dave]

Have just emailed my dad

 

He's into this sort of stuff

 

He's actually a metrologist, but was formerlly an electrical engineer.

 

Will see what he says and get back to you....

 

It's odd that he is a meteorologist as this is exactly what I want the antenna for.

 

I should be able to get images like this, once it's all up and running:

 

http://web.ukonline.co.uk/phqfh/Retro/Dave%20Cawley.jpg

[Rooster-UK]

The radio amateurs examination has altered dramatically since I took it. It's all multiple choice questions now.

When I took it, it was a 3 hour written examination covering maths, licence regulations, aerials and propogation, interference and suppression and I think the other was circuits.

The circuits question that I got was "Draw a circuit for a TRF (straight) receiver, suitable for reception of signals on the short-wave bands." An easy one!

 

That was 40 years ago!

[Rooster-UK]

Reception will always be better with an external aerial. For a start, you gain an extra few feet in height.

The other aspect is that, if mounted inside the roof, there will be some attenuation of the signals when the roof is wet.

Even more so if the roof is covered with snow!

[dave]

I thought as much - that's why I got the (incredibly long) pole to hoist the antenna on. I'm going to strap it to the railings and run the cable through the window - I just hope I have tolerant neighbours!

[HSBCrusher]

would that need planning permission?

[dave]

This diagram of the same might be more use to someone that understands it; I don't ;-)

 

http://www.jcoppens.com/ant/qfh/img/v2/012.jpeg

[dave]

would that need planning permission?

 

I suspect I'll find that out after I have put it up!!

[Rooster-UK]

I used to have a 6 over 6 slot-fed yagi for 2m and above that, an 18 element parabeam for 70cm.

Also had 5/8 whip aerial mounted against a groundplane for working mobiles.

 

Never had any complaints.

[Rooster-UK]

would that need planning permission?

 

Unless the regs have changed, no.

[daviet1976]

Dad is on holiday apparently.

 

And he's a metrologist (measurement) not meteorologist (weather).

 

He's now head of QT for Michelin in Europe. Or something.

 

I lose interest easily.