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Old 1st Apr 2013, 12:57 pm   #81
turretslug
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Default Re: The Future of Short Waves

Thanks, arjoll- interesting stuff, and it reinforces post 68's point about the aerial system being the difficult, expensive and potentially dangerous bit.
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Old 1st Apr 2013, 1:21 pm   #82
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Default Re: The Future of Short Waves

Quote:
Originally Posted by arjoll View Post
Sort-of related, this thread prompted me to have a quick look at the RNZI website, and this technical info may be of some interest.
You can see where the cost (in this 'know the cost but not the value' world) mounts up running the HF services - even utilising the most modern and efficient equipment the plant and land costs are immense. Conversely, to mount an 'International presence' on the internet what does a radio station need - a few 'U' of 19 inch rack space. Sad I know...
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Old 1st Apr 2013, 1:47 pm   #83
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Default Re: The Future of Short Waves

The 750 Acres needed for Skelton's antennae was compulsorarily purchased during WWII, and is currently let for grazing sheep, amongst the groundwork. But the touble with land is that they're just not making it any more...
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Old 1st Apr 2013, 2:34 pm   #84
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Default Re: The Future of Short Waves

The wind turbine folk know about tall, slim, low maintenance, self-supporting hollow masts, they even have internal access lifts and are almost "off-the-shelf".

Fantasy thought exercise- what would the technical possibilities/costs be of putting up one of these without generator head but with 12MHz radiator slots cut into it? I know it's after mid-day...
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Old 1st Apr 2013, 5:41 pm   #85
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Default Re: The Future of Short Waves

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what would the technical possibilities/costs be of putting up one of these without generator head but with 12MHz radiator slots cut into it?
And why not have the generator too? Given a properly spaced number of them it would make a good directional array.
 
Old 1st Apr 2013, 7:01 pm   #86
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Default Re: The Future of Short Waves

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Originally Posted by merlinmaxwell View Post
And why not have the generator too?
Well, maybe being a bit cautious in order to avoid any "Tacoma Narrows"-style laughing stock,

1) I assume that cutting slots in the hollow metal tube would affect torsional rigidity/strength as regards the windmill bit on top.

2) QSL cards might have a common theme- "good signal strength, great programming- but there was an annoying slow thump, thump, thump fade on carrier level"...

Still, where there's a will, there's a way
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Old 1st Apr 2013, 7:45 pm   #87
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Default Re: The Future of Short Waves

At the GMDX meeting a few years ago, there was a talk on the other end of some of these broadcasts... Quartz hill, NW of Wellington was a receiving site for broadcasts from the UK, which passed the signals on for local transmission as well as HF to the rest of the Pacific.

It was an amazing flat plateau, full of wooden poles bearing Rhombics and long wires.

The local amateurs got the use of the place for a while, but it was eventually sold off to become a wind farm. The rhombics had to go.

It's at Lat -41.253429 Lon 174.69685 if you fancy a shufti.

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Old 5th Apr 2013, 10:27 pm   #88
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Just been looking at the new summertime shortwave schedules. Very painful deep cutbacks of the BBC World Service but nice to see a full comprehensive schedule from R Australia. Other services such as VoA,R New Zealand and the recently pruned VoR are still hanging on. So still some interesting stations available to European shortwave listeners.
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Old 9th May 2013, 3:45 pm   #89
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Default Re: The Future of Short Waves

I have just been reading this thread, the doom and gloom being put forward is heart breaking, it makes you want to get rid of all your old Tx/Rx Recievers and become couch potatoes.

If we went to war last month I think the powers to be would have asked us all to send our gear to Bletchley park, as a nuclear strike or an astreroid crashing into one just one of the many satellites could put all satellites out of action due to the fallout of debris and they would only be left with our valve stuff, and maybe that's the reason that all other countries are using up all the air space for AM.

So I thought I would put in my program listings for the past month to the present day.
I don't think AM is dead or dying just yet and only time will tell.
Here are some of the programs I received on my AR88D.
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Old 9th May 2013, 8:14 pm   #90
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Default Re: The Future of Short Waves

Very interesting, long live the AR88! Must fire mine up again after many years of rest.
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Old 10th May 2013, 8:04 pm   #91
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Default Re: The Future of Short Waves

"OH" HamishBoxer GM8JET.

Shame on you for leaving it so long, you should get it up and running a.s.a.p, as the longer she sleeps the more it will cost to wake her up.

It took me a long time to aquire this old girl, and put her back to a healthy condition now she has rewarded me with great enjoyment.

I think I would go crazey if she was (turned off) for more than a day, she gets switched on every morning about 9.00am and goes (off) to sleep when I decide to go to bed.

She also acts as a heater for the room as I have her uncased so I can watch the movement of her tuning Ribs and the glow of her Dial/Lights.

She gives me all those stations and more yet to Log, via the unusual Aerial I put up to feed her.
PS:the two wires are connected together at the junction box.

Here is the Aerial, comments welcome. Gezza123
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Old 10th May 2013, 8:44 pm   #92
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Your aerials look exotic but for LF/HF work they look like a short piece of wire to me. How have you coupled the 'junction box' to the radio via the 10 feet (assuming the drawing is to scale) to the radio. I am sure it could be bettered, I don't want to sound fierce but a good antenna system is the key to good radio.
 
Old 10th May 2013, 10:15 pm   #93
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Default Re: The Future of Short Waves

Hi, Merlinmaxwell

I coupled the junction box to the radio via RG-59 Coax, yes the drawing is to scale as I dont have a big garden.
I was going to make Delta Loop but then I would have to make a 4:1 Balun.
Also I looked at geting a tuner of some king but the price is prohibited, so I just put up a long wire at first and then thought I would put up another one as a sloper and just switch them, or connect them as a Vee Doublet, but the wiffy did not want any wires hanging about so I just connected them together in the junction box and coaxed to the radio room and the results speak for its self.
I am listening to india Overseas radio in english fortnightly magazine 22-08 on 7.560Mhz while typing this @ S10

I dont know a lot about Aerials so any help appreciated Gezza123
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Old 11th May 2013, 12:42 pm   #94
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Default Re: The Future of Short Waves

You will lose a lot of signal (unless by chance the aerial matches the coax) merrily joining the areial to coax. A good compromise is using a 'magnetic balun' this increases the areial side to about 900 ohms. I use a Wellbrook loop, expensive but marvelous.
 
Old 11th May 2013, 10:39 pm   #95
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Default Re: The Future of Short Waves

Hi Merlinmaxwell.thanks for your reply.

I have just looked at there site, they look an excellent piece of kit but i'm affraid they are out of my budget, I could buy another Receiver for the price.

Googling loop Antennas brought up loads of them to make, so I may try and make one when the weather up here gets better.

I have now just connected the receiver as a balanced input as per manual and the (S Meter) has shot up to near full scale and the noise level has dropped appreciable, the ssb bands are incredible and I have to really turn down the RF.Gain or you can hear them next door, the gain of these sets are really excellent.I will now have to retest all my previous (S Meter) readings.

Also I downloaded some simple information on (Building and tuning Dipoles) so will have an early night and do some study.

Thanks again for your input. Gezza123
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Old 19th May 2013, 9:04 pm   #96
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Default Re: The Future of Short Waves

Hi Guys, some help questions please.
This is a follow up to my (dedicated Receiving only Aerial )

Defying Logic.
My long wire Aerial has now been modified, it is two lengths of wire connected together as a (Vee) one black @ (33.2ft and one red @ 36.8ft) totaling 70ft, they are then taken to a junction box on the wall.
A 10ft coax lead is then taken through the wall to the receiver, the top end of the coaxial braid in the box is taken down the wall to a copper rod in the ground and then to an outside water pipe.

Note:The Ground on the receiver is not connected to the coax braid, (Link open), plus I have tried connecting the (Link) to the ground on the radio and there is no significant difference.

I am receiving all the AM/Amatuer bands from (10 mtrs to 160mtrs), plus (MW) so why does it work so good, when there is no centre dipole connection.
IE --------------| |----------------

Questions
(1) where is the dipole, as there is no insulated feeder connection to the wire.
(2) Is it the braid going down to the ground rod acting as half a dipole, and should it be disconnected from the ground for any safety reasons.
PS: I have tried disconnecting the ground from the earth rod and it makes no difference to the signals levels.

If I disconnect one of the Vee wires the signal level drops and I loose some Amatuer bands with noise.
(3) Is this Aerial working as a (quarter or half wave) on harmonics.
(4) And is it correct to have a direct connection feed to the two wires.

Sorry if I am a bit thick here, but it dont seem to add up with the formula for a wire fed multiband antenna.
After a bit of study and a headache, I have tried to work it out, I have now got some sort of voltage curves for this multiband which gives me such excellent results, with no insulated dipole feed connection.
Hence my confusion.Thanks in advance Guys for any input.
Gezza123.

picture 1.how its connected in the box/receiver.
picture 2. Voltage curves.
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Old 19th May 2013, 10:28 pm   #97
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Default Re: The Future of Short Waves

From the first picture it looks as if you are just connecting a couple of decent-lengths of wire joined near the centre to the inner of the coax, and using it against earth. That will be fine for a general receiving antenna, (the rx AGC more or less smoothing out the effects of impedance variations/mismatches across all the bands), but its real performance would show up then trying to match it to a transmitter on various bands, or using an antenna analyzer on it!

It's been mentioned on this forum more than once that connecting an unmatched/non-resonant antenna to a receiver via coax of any significant length (say more than a metre) is in effect just putting a capacitor from the antenna wire to earth, thus losing a significant amount of signal in the capacitance of the coax. It will work well where the impedance of the antenna is at or close to that of the coax, but this will only occur on a few parts of the bands.

Not only that, but all domestic vintage sets I've ever seen have an antenna input that is just a non-coaxial terminal and having no claimed input impedance. It won’t even match to the coax impedance.

So, the only way to use coax (usually 50 ohms impedance) as the screened antenna cable to a receiver is:

1) If the receiver had a coaxial antenna input.

2) Use coax to connect to the centre of a dipole. (Note: One cut for say 3 MHz will also work well at 9 MHz, 5 MHz and 15 MHz, etc.)

3) Match the antenna to the coax across a wide frequency range with a high dynamic range RF amplifier. The this is what the Wellbrook loop does.

4) Connect the length of antenna wire to a balun. This is typical, but I think is quite expensive for what it is, and many home-brew designs are available by just Googling. (I’d be very wary of the claim that from 100 kHz to 40 MHz it will convert a random wire’s impedance down to 50 Ohms. Oh, if only such a device existed!!) Such a balun is a compromise; it’s an attempt to reduce the miss-match losses if a coax feed is essential, i.e. if a random wire has to pass near sources of radio interference on its way to the radio as it comes into a building. In my experience a balun used like this really only works reasonably well if the antenna is 10 metres or more in length. (This is probably an "un-un", not a balun anyway!)

5) Use an antenna matching unit between receiver and antenna.

But to be honest, for listening to shortwave signals on an ordinary domestic shortwave receiver, you probably won’t get a huge improvement over just sticking the antenna wire in the antenna socket, and using a good earth. The main problem is finding somewhere to put the antenna that won’t pick up too much noise from electronic and electrical devices.

Regards,

Ian
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Old 22nd May 2013, 9:52 pm   #98
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Default Re: The Future of Short Waves

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Originally Posted by Ian - G4JQT View Post
From the first picture it looks as if you are just connecting a couple of decent-lengths of wire joined near the centre to the inner of the coax, and using it against earth. That will be fine for a general receiving antenna, (the rx AGC more or less smoothing out the effects of impedance variations/mismatches across all the bands), but its real performance would show up then trying to match it to a transmitter on various bands, or using an antenna analyzer on it!
First thing, is thank you for the above information.
You are correct in what you said in the above, and I did not realise the coax would be acting as a capacitor, you are also correct in that I will loose some signal strength, but the amount is very low and makes no difference to the quality of the programs in that all I have to do is put the RF control up a small amount to get the levells back.

I then tried to connect it as a dipole with the centre Insulated and found that it was very directional an did not receive all the present stations I had, so I then went back to how I had it in the first place and removed the earth from both ends of the coax IE (no caxial connection) and took the earth on the receiver directly to the outside ground, the gain went up but there was a lot of static and the voices were lost in it.

So all in all, I have still got two lenghts of wire isolated at each end and connected in the centre with the centre of a coax connected to the wire (no isolated feeder connection), and the braid going down to the outside wall to ground rod.
I am not complaining about the reception, I was confused as to why it worked so good when the books tell me different, maybe I am just fortunate where I live and the signal levells are good.
PS: It will never be used on any transmitter as I am not licenced, and will only be used on one dedicated receiver the AR88D.
Thank you again for your input, regards Gezza123
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Old 1st Jan 2015, 3:26 am   #99
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Default Re: The Future of Short Waves

The Voice Of Russia is gone from SW now, the BBC is hard to hear in Australia, I can't find Voice Of America at all, and it's China Radio everywhere! It's a pity about Russia; the Australian Government is giving them the cold shoulder and they're forced to pay for local newspaper supplements to reach the public here! I shouldn't feel nostalgic about the Cold War, but listening to radio propaganda from the USSR, the USA, and China on shortwave is a fond memory for me.
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Old 1st Jan 2015, 4:55 am   #100
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Default Re: The Future of Short Waves

I started out listening to broadcast stations on 31m, with a two-valve TRF with its long wire aerial and headphones. But now I get my fix via my internet radio (which is relayed throughout the house), and I'm listening to overseas stations (mostly US and Canadian Public Radio, which are conveniently time-shifted) many more hours per day than I ever did with my TRF. I really enjoy the alternatives to the Beeb, especially in getting different views of world events and politics. Not as romantic as the valves, but very functional .

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