In message , Brian Gaff writes
Presumably only because the unwanted stations (local pirate broadcasters?) were vertical.
So horizontal was just as bad as vertical?
A horizontal halo shouldn't have any narrow notches. Its pattern should be a slightly squashed circle. [If I remember correctly, it's slightly better 'off the sides' rather than 'front' or 'back''.]
No - A horizontal halo has a significant notch opposite the feed point.
My half-wave dipole will be laterally offset by about that amount, to clear the TV aerials. However the bottom half of the dipole will dangle below the bottom of the mast, which amounts to the same thing as what you describe, I imagine. :-)
Anyway this became GSETDI, not DIY, when I discovered that my ladders don't quite reach high enough for me to work comfortably. Still, working out how to DIY leaves me better equipped to supervise someone else doing it.
In message , charles writes
I'm pretty sure that's not what the polar diagrams I dug out some time ago indicated. I'll try and find them again.
In article , Ian Jackson scribeth thus
It will have a variation on that theme depending on the spacing and if the mast mount has any resonant properties at the frequency of interest...
If you have a 2 story house with no large buildings around then a multi element in the loft will work quite well where you are...
In article , Ian Jackson scribeth thus
Its piss poor in most any direction or plane of polarisation, in fact a waste of good Ally;(...
In article , Ian Jackson scribeth thus
Well how were they obtained?, I bet the makers didn't do free field testing i.e. away from large reflective objects or an EMC model of it..
Even pro makers aren't that fond of such detail, mind you the mast mount does have quite an effect..
In message , tony sayer writes
They may not have tested the polar diagrams at all (or maybe done only a rudimentary check). However, nearby reflective objects will affect the polar diagram any omnidirectional aerial.
A relatively thin vertical metal mast won't have much effect on a horizontal aerial (even one encircling it).
Anyway, have a look at Fig22 (and 25).
and Fig26
Also, have a look at the link
OK, the patterns are NEC modelled, their construction is not quite the same as domestic FM halos, but the patterns will be the same.
I do have a two-storey house and no large buildings around. What I don't have is a loft, not in that part of the house anyway. :-)
Flat earther then;(..
Yep good site that one, have to look at the eggbeater for another application. However there're all well made with proper matching tuning unlike the ilk of domestic ones.
I can see how that came about its much simpler to mount fulfils the request for an "FM" aerial and its cheap...
Never did get on with that 4nec2 prog. We usually use NEC4WIN95VM which is now old and unsupported but is very quick to use then if needed for Ofcom approval transfer the file to NEC WIN Pro..
In message , tony sayer writes
It's unlikely that the matching accuracy will affect the polar diagram, although the lack of a balun will skew things a bit. I think the original concept of the folded dipole format was to help increase the bandwidth (due to its chunkiness) and to increase the feed impedance (which would be a lot lower than the (say) 300 ohms of a straight folded dipole. My halo (bought around 30 years ago) is really intended for 300 twin feeder - and the instructions are very vague about attaching coax. I've never really used it in anger.
I have to admit that I've never really tried to use these programs myself.
But are we agreed that the halo is probably not quite as useless as others would have us think?
In article , Ian Jackson scribeth thus
Yes well think of the Balun as a matching component. The BBC back in the day they did some experiments and found that the unbalanced part of the usual domestic aerial connection made the feeder cable part of the aerial with undesirable results..
It does . We use a pretty standard Jaybeam 7051 folded dipole and have used them in multicoupler arrangements to run paging Tx's at 153 MHz and RX on the lower part of band 3 at around 180 odd as well as the more usual 165 MHz odd channels on the same aerial !..
;!..
Don't unless you have to, theres quite a learning curve;!.
They are needed when planning TX aerial installations where the surrounding metal work can impinge on the radiated field. I did one once it all looked quite simple but there was a 16 dB pattern variation over a 360 deg plot!..
Well any old bit of wire acts as an aerial .. but some bit's of wire work better than others;)..
Indeed. ;-)
Which reminds me of an odd problem with a cheap car radio I fitted to my caravan.
The caravan has a built-in aerial, which appears to be one of these:
The radio's FM performance was not good, so I started to look a little more closely. The aerial has a power feed, presumably for some kind of internal amplifier. Energising this made no apparent difference either way.
What was odd is that if I pulled out the plug at the back of the radio, so only the centre pin of the aerial connector made contact, but not the outer, I got much improved reception.
I wondered if the aerial was seriously faulty, so connected up a simple wire T dipole that I had around, and that was actually a bit better. Sadly, in some locations this still wasn't very good.
Another chance discovery was that if I plugged a 2 m mini-jack lead into the auxiliary input socket, things were much better, and I now use this method when reception is poor, though it does seem to need waving around the caravan each time to find the best signal.
Am I wasting my time with a too-cheap Tokai radio? I do have a back-up aged portable, and it never has a problem finding a signal.
Chris
Sounds like it was a pants design or it's gone tit's up. A simple vertical quarter wave on a caravan roof will be fine in fact you'd be hard pressed to get better.
It looks like the radio aerial on a Taxi but a bit longer.
I can give you the address of a supplier if you want..
I'd be interested, but the biggest problem would be installing and running the cables without internal access to the roof. Caravans aren't really built with that kind of access in mind.
Chris
Have a rummage thru this lot, they may well do a "gutter mount" but either way if you can get something like that on the go it should perform really well:)..
Cheers..
In message , tony sayer writes
If the roof was steel, an FM band quarterwave on a magmount would work very well. This could be a cut-down 5/8th wavelength for the amateur 2m or for CB - with the bottom spring coil removed, and the actual whip cut to around 75cm (as 100MHz is a wavelength of 3m). However, I expect the caravan is aluminium.
If a magmount is suitable, and if the coax cable is fairly thin, it could probably be brought in through a hinged opening window (and the window closed on it). It would probably still be watertight.
Course you can always pop rivet glue or both a sheet of galvanised plate onto the roof we have to do this on modern farm vehicles. These almost without exception nowadays are all fibre glass which is a real PITA due to the amount of onboard electronics in a tractor now, the interference levels can be quite high, that "missing" metal roof is a bad omission;!...
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