Blatant advert for C&I forum to discuss all things.

[Keygrip]

Heard from a reliable (?) source that this forum was to discuss all things electronic....but also heard from another that it doesn't advertise it.

So I did.

[Saab Dastard]

Well I've asked for advice on plasma TVs...

SD

[BOAC]

Into cryptic crosswords too?:) This forum has for a long time kindly 'tolerated' questions on TV/DVD recorders/Mobile phones and the like. What did you have in mind?

EDIT; He should know :ok:

[Loose rivets]

I got told off once for talking about tellies. Then I noticed the teller-offer only had two posts, and half of those were spent...erm, telling me off.:}


I suppose one could say that anything that could connect to a computer should be allowed, so that's everything, right?

[frostbite]

I wanted to discuss electronic organs but I was told to keep sex out of it.

[Jofm5]

I think some do not know we even exist, here is like the dusty corner of the forums where "we dont speak to those" :}

Seriously on that point it was kind of highlighted with the "Cannot Connect" issue where a few threads had posts about the issue when it was already being discussed here (when possible).

Perhaps we should take the word "issues" out of the thread title and replace it with discussions as you dont need to have an issue to want some advice.... e.g. recent threads on iphone apps etc.

Oh and whilst I am bleating.... we dont really have a place for discussion of photography - after my disappointing pictures taken at Biggen Hill I have spent a small fortune on digital SLR/lenses using the advice from others who posted their pics - however to get that advice was thread drift in those forums - perhaps we should change the forum title to be more inclusive...

Just my 2p worth :ok:

[PPRuNe Towers]

I'd welcome threads expanding to include AV and photography.

Let's face it there are plenty of well established forums for those subjects elsewhere though just as there are for the main topics already here. To differentiate us from them I'd like a bare knuckle approach though:eek:

Anything based on our ears and eyes doesn't respond well to those who just like to rattle off specs or just rave about equipment plucked from some internet mail order joint on price.

Further I'd propose lifetime bans for anyone holding forth rearding their iPod/iRiver/Zen/Sony who uses the supplied ear buds. Instant electronic death for those lauding large flat screen TV's still on their 'grab the suckers,' showroom settings would of course be a de facto condition. And finally I'm still dwelling on how to deal with those using cable, of any sort, that came in the box.

Other than that I've no strong feelings on the subject and it's entirely up to Saab who is immolated at his hands:E

Rob

[Sprogget]

Personally. I would institute public floggings for the insecure at heart brigade who like to come here & say; I'm more I.T. than you. No, I'm more I.T. than you. No names or pack drill, but I know who I have in mind.:E

[Flintstone]

This is a gadget forum too? Excellent! Off to start a thread.




"Dear Geeks.......nerds...........ummmmmmmmm"

[Keygrip]

My wurk is done here.

<Goes off to wash his paws>

[Keef]

Eee! Can I ask about this problem I have with the balun on my HF antenna?

Wouldn't want to be cast into outer darkness...

[Jofm5]

You could keef.... and I would probably answer lol

As a kid I was very much into HAM radio (yep I am a nerd/geek/wonk) although I never did get my licence I did go through aeriel design etc so could give some advice on swr readings and how to achieve better.

I guess saying this is going to downgrade me from "sad git" to "really sad git" lol :}

[Flintstone]

Hydrogen or helium?

[Keef]

Well, doctor, it goes like this: we live in a "no outside aerials" neighbourhood.

I have a (carefully measured) set of dipoles in the attic. They cover 10, 14, 18, 21, 24 and 28 MHz. They are splayed out at approximately equal spacing, so each radiates in a different direction. They are all on one feeder, with good quality coax, and connected via a very good quality balun.

They work well on all bands except 18 MHz. SWR in the (CW) band centres is close to 1:1, rising as the frequency increases. On 18MHz, the SWR is 5:1 and the matching unit doesn't like it one tiny bit. I've checked several times, and the dimensions of the dipole are spot on.

The separate dipole for 50MHz works fantastically well, but that's the clever broadband design made of 300 ohm ribbon feeder.

Power level is around 150 watts up the coax (except on 18 MHz, where the protection circuit trips in).

What's to do?

[Jofm5]

I've checked several times, and the dimensions of the dipole are spot on.



Are the dimensions your checking the electical dimensions or the physical dimensions ? The optimum aerial will be equal to the wavelength electrically or a dimenion thereof - e.g. a quarter wavelength will work much better than a misc length - some aerial manufacturers will "load" the antenna by adding a coil to make up the electrical length. In a permanent fixture you will adjust by physically cutting the antenna to length based upon SWR value.

If I recall correctly a SWR of 5:1 is totally unacceptable and my meter only ever went up to 5.... Typically if your SWR is incorrect you have an insufficient ground plane (your signal is being reflected back to itself) - try placing some sheet metal under the dipole and grounding the outer brade of the coax from 18mhz dipole to that - for best results again you will want a factor of the wave length on your ground plane for what your trying to balance.

Edited to add an inline amplifier after the SWR meter will distort your readings so balance it before adding an amplifier.

[green granite]

Cross coupling effect between antennas? Or a bad connection? Do you have or can you borrow a noise bridge to directly measure the resonant frequency without the balum?

[Senior Pilot]

we dont really have a place for discussion of photography

There is a long running thread on Rotorheads: How to post photographs (Incl photography tips from the gurus) (http://www.pprune.org/rotorheads/145070-how-post-photographs-incl-photography-tips-gurus.html) :ok:

[Saab Dastard]

Photography is almost worthy of its own forum!

There's frequent discussion in AH&N, obviously regular posts in the 'choppers* forum, and it comes up in here from time to time.

SD

*just to wind up SP

[frostbite]

Keef, if the roofing felt under your tiles has a layer of foil, you're not going very far and certainly not going to achieve 1:1.

[Keef]

Are the dimensions your checking the electical dimensions or the physical dimensions ? The optimum aerial will be equal to the wavelength electrically or a dimenion thereof - e.g. a quarter wavelength will work much better than a misc length - some aerial manufacturers will "load" the antenna by adding a coil to make up the electrical length. In a permanent fixture you will adjust by physically cutting the antenna to length based upon SWR value.
'Tis the electrical dimension. It's a half-wave dipole (two x ¼-wave lengths of wire). All the dipoles on there are the same. I did the calculations very carefully when I cut the wire and installed it. All the others matched spot on, first time.

I'm wondering if there's some strange harmonic relationship between the 18MHz dipole and one of the others, but I can't work out what that would be. I know that 21MHz signals react "interestingly" with 7 MHz antennas, but I can't see any such relationship between the ones I have on that feeder.


If I recall correctly a SWR of 5:1 is totally unacceptable and my meter only ever went up to 5....
5:1 is certainly not good - certainly for a 50 ohm dipole that should be resonant. However, my meter goes all the way to infinity.


Typically if your SWR is incorrect you have an insufficient ground plane (your signal is being reflected back to itself) - try placing some sheet metal under the dipole and grounding the outer brade of the coax from 18mhz dipole to that - for best results again you will want a factor of the wave length on your ground plane for what your trying to balance.

I don't think so. There shouldn't be a ground plane under a dipole. In theory the dipole should be in "free space" with nothing else around it.


Edited to add an inline amplifier after the SWR meter will distort your readings so balance it before adding an amplifier.
Not much point in putting an SWR bridge between the exciter and the amplifier, surely? If you designed and built them correctly, then the matching should be OK. Otherwise, it will show the mismatch at 50 ohms between the output of the exciter and the input circuit of the amplifier.

This reading is the SWR on the coax feedline to the dipoles. After the SWR bridge comes coax, then the balun, then the dipoles. All bands except 18MHz show 1:1 or close.

Meanwhile, I just don't operate on 18MHz.

[Avtrician]

May be your 18 meg signal is trying to feed the 21 and 24 meg dipoles (all are multiples of three??), tho theory says it should use the wire length most suitable for the freq.

RF is a strange animal.

Once had the HF in a Huey that was working perfectly from 3 to 30 meg. On a servicing we found the longwire feed had broken at the fuselage feed thru. That was repaired and the HF went back to its usual crappy operation:uhoh::uhoh::uhoh:

[Keef]

Keef, if the roofing felt under your tiles has a layer of foil, you're not going very far and certainly not going to achieve 1:1.

I did wonder that, but the others all get close to 1:1, and the little 50MHz one goes like the clappers (working VKs when the band was open).

Maybe there's a load of resonant stuff in the roof insulation ... hadn't thought of that.

[taxydual]

My God! I thought the computer guy's were the only one's to talk Geek!

You hammers are in a league of your own!

(Fascinating stuff though. Dipoles, foil covered roof felt, etc etc. Strangely addictive.)

Hope the problem is solved.

[green granite]

My God! I thought the computer guy's were the only one's to talk Geek!

You hammers are in a league of your own!

Just wait until we start bringing in peanut butter to the equation. :E:E

[birrddog]

You hammers are in a league of your own!

(Fascinating stuff though. Dipoles, foil covered roof felt, etc etc. Strangely addictive.)
Just wait until they start describing their foil hats and associated ham conversations :E

[birrddog]

Seeing that Keygrip gave us some grace in the scope of this thread... :E

What are the thoughts on a PPRuNe Wiki?

Organize photo's/links/snippets of knowledge that we have garnered - would clearly need editorial management, something like Wikipedia, but could be a good way to manage the knowledge hidden in the depths of our archives.

[Keygrip]

Noooooooooooooooo - don't blame me. It wasn't me.

I was at home watching the TV, all alone.

I can't remembear what was on.


It was either Colonel Mustard, in the library, with a lead pipe - or flintstone in the attic with an 18meg dipole.