1300 Discone-HF

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1300 Discone-HF

Postby mike43 » Sat Nov 19, 2016 7:19 am

Don't want to sound like a schoolboy here but this morning I thought I would plug my Discone 1300 antenna from my scanner into my Tecsun S2000 radio to see what I could receive. The first station I came across was Radio Habana on 5040 MHz. Then I disconnected the Discone and attached the random wire antenna. There was a lot of difference in signal clarity with the Discone and a lot more noise with the random wire and no Radio Habana at all with using an antenna matcher. Can anybody on the forum give me some pointers as to how much I would be able to receive with the Discone as opposed to the random wire antenna. Is it something to do with the Discone being a wide band and omni-directional antenna or is it to do with something else. I also picked up Shannon Volmet. At 6.20 am also picked up BBC Ascension islands on 5875 MHz, did not understand what they were talking about as it was in french, so something must be working more with the Discone. Thanks. Mike
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Re: 1300 Discone-HF

Postby G4RMT » Sat Nov 19, 2016 10:08 am

Your discone is simply another random antenna - it's a bit of wire in the sky, but with HF there are other factors that appear to make things different. When you change aerials and notice the background noise go up and down, remember the physics. if the background noise goes up, you are receiving more signal. Noise on your frequency means the aerial is capturing more RF. The snag sometimes is that the RF you are capturing is coming from a different direction to the signal you would like to hear. If you live near a noise source - like a busy city, full of EM noise, then your weak signals need to get above it. Some of those shortwave broadcasts use mega power, so leap out above the rest. Moving to the discone (a worse HF aerial) means you still have plenty of the broadcast station, but less noise. Subjectively it sounds better. It's just not going to be able to 'collect' the weak signals. When you use an ATU, the background noise leaps up when it gets to resonance. The actual capture ability of the aerial hasn't changed at all - just the matching to the receiver. I've been doing some listening trying to compare different aerials - with an aerial switcher and using SDR. These really are pretty rotten radios with loads of spurious signals that they generate internally, plus my computers mot lots of RF rubbish which then gets picked up by the SDR and displayed on the screen. It doesn't really mattter I suppose, but I noticed that switching to the airband VHF aerial removed lots of these computer generated noises. Great aerial, but then I noticed the weak signals perfectly listenable to on the other aerials, with all the sproggies was missing. A much quieter aerial, interference wise, but actually just not a good aerial anywhere other than air band. I think this is what you are finding with the discone on HF. My discone is pretty rubbish on HF - just hopeless at capturing the weak stuff. My ATU lets me tune it for transmit too - but the long wire I used to have up till the tree trimming people came, was much better - it picked up much more noise! Which is of course, just unwanted signals.
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Re: 1300 Discone-HF

Postby mike43 » Sat Nov 19, 2016 4:10 pm

My thanks to G4RMT regarding the antennas. The random wire does give me more noise but I am still pleased with getting Radio Havana and BBC Ascension Island on the Discone. I often wonder how much of the noise comes from inside the house with the random wire. The random wire is about 50' long and about 20' up with about 20' more going around inside the loft then down through the bedroom ceiling. It is at an angle laterally outside with one part going roughly E to W and the other leg roughly going NE to SW. It comes into my bungalow at gutter height then around the loft and down through our bedroom ceiling to the receiver. Is there a simple test to work out if there is RFI coming from inside the house. I have found that early mornings (4am) are a lot quieter than in the daytime. I was also wondering if the coax (RG213) from the Discone made the receiver quieter because of the shielding properties??. I have also sort of looked casually at ways of reducing some of the noise if I can identify where it is coming from. I have read some bits about baluns and ununs (still trying to understand about them) if they reduce some noise. Years ago I had a Realistic DX 349 for SW listening. I found a secondhand book in a charity shop about transistors and other things. One of the parts I got interested in was what I now know to be an ATU. I got a piece of broomstick about 6" long and did the windings and tapping points. This I have also learnt was an inductor. I was amazed one day just moving the little crocodile clip along the tapping points to get a hit from the South Island New Zealand weather forecast. I called the wife and the kids to make sure I was not hearing things. The next day I tried it again and it worked. To my dismay a few days after that I spilt a glass of water on the receiver by the bed and mullered it altogether as it was switched on. I probably could have had it repaired. I havn't had any hits that way recently as I suppose they have all closed their SW stations because of the cost. What with expenses and kids and all that I lost interest until this year. I have since built a more sophisticated ATU and I get the urge to build an even better one if it were possible. My laptop doesn't seem to make much noise as I have switched it off and it doesn't make a difference. If the scanner is on it seems to pick up noise from it or the wall wart. Anyway apologies for the epistle, and thanks again for the help. Mike
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