|
how to combat noise?
Here is the situation: have an inverted V for 80m and noise level is
S9.. Not good really.... I did the next logical step and tryied magnetic loop for 80, reciving only, indoors. That antenna failed awfully.... Noise was S8 but I could not hear stations that were 9+10 on inverted V. Did'nt matter how I tryied to rotate the loop, all there was is noise. So, indoors is right out.. All that is left is roof. And that makes my situation interesting. It is a big house with new metal roof, electrically bounded. 10X60 metres, 17 metres hight, angled about 20 degrees. Big ground plane indeed. All I could figure is to get some sort of antenna to the roof so the roof would act as a shield from all the noise generated inside the house. That antenna would be for receiving 80m, can be narrow band (50KHz ok). NVIS pattern would be prefered. So, I have huge ground plane, lots and lots of noise below it, and need a receiving-only antenna. What would you do in this situation? Loop, magnetic loop, dipole, short beverage perhaps??? Any advice is welcome. Right now I cant even figure if that roof is acting like a shield or a big noise-sucking antenna.. Andrus |
how to combat noise?
Andrus wrote:
"Any advice is welcome." Andrus has a bonded metal roof 10x60 mtr which is 17 mtr high. An automobile contains radio noise within its engine compartment by using a metal cover. A roof does not completely cover and surround noise sources within a house, but if it is effectively grounded at the receiving frequency, the roof can help block transmission of noise into an antenna mounted above it. A dipole is balanced and slightly directional. This helps avoid noise pick up. A balun at the antenna feedpoint helps keep the antenna balanced and allows connection of coax to run through noisy areas without pickup. A low dipole over a reflective surface has a high-angle radiation pattern, excellent for NVIS. With an agile antenna tuner you may be able to hear and work stations within several hundred miles. Best regards, Richard Harrison, KB5WZI |
how to combat noise?
andy wrote:
Here is the situation: have an inverted V for 80m and noise level is S9.. Not good really.... Any advice is welcome. Right now I cant even figure if that roof is acting like a shield or a big noise-sucking antenna.. If you can somehow get your inverted-V to a horizontal position for testing, you might be plesantly surprised. At my QTH, going from an inverted-V to a horizontal dipole at the same center height lowered my noise level by 2 S-units on 40m. -- 73, Cecil http://www.qsl.net/w5dxp |
how to combat noise?
A couple of antennas that work well at this qth...typical urban environment
with elevated power lines and assorted other racket. Run a big horizontal loop around the place..feed with coax for recieve only or twin lead plus transmatch for recieve/transmit. This is the one I am currently using. Previous antenna...which also works pretty good was a 100 ft bowtie doublet. I used to run the doublet with vertical drops on the ends, but found that any vertical drop on the ends of the antenna really picked up the racket. If I left the vertical drops off the ends, I never did like the way the Johnson Matchbox was coupling with the feedline...450 ohm...on 80 meters. I would up with 10 ft end drops, then running a wire from the bottom of the end drops back up to the feed point. Tuned a lot better, broad banded and rejected noise pretty well. "andy" wrote in message oups.com... Here is the situation: have an inverted V for 80m and noise level is S9.. Not good really.... I did the next logical step and tryied magnetic loop for 80, reciving only, indoors. That antenna failed awfully.... Noise was S8 but I could not hear stations that were 9+10 on inverted V. Did'nt matter how I tryied to rotate the loop, all there was is noise. So, indoors is right out.. All that is left is roof. And that makes my situation interesting. It is a big house with new metal roof, electrically bounded. 10X60 metres, 17 metres hight, angled about 20 degrees. Big ground plane indeed. All I could figure is to get some sort of antenna to the roof so the roof would act as a shield from all the noise generated inside the house. That antenna would be for receiving 80m, can be narrow band (50KHz ok). NVIS pattern would be prefered. So, I have huge ground plane, lots and lots of noise below it, and need a receiving-only antenna. What would you do in this situation? Loop, magnetic loop, dipole, short beverage perhaps??? Any advice is welcome. Right now I cant even figure if that roof is acting like a shield or a big noise-sucking antenna.. Andrus |
how to combat noise?
Denton wrote:
I used to run the doublet with vertical drops on the ends, but found that any vertical drop on the ends of the antenna really picked up the racket. Same as at my QTH. Anything vertical is noisier on receive. If I left the vertical drops off the ends, I never did like the way the Johnson Matchbox was coupling with the feedline...450 ohm...on 80 meters. Because your feedline length was somewhere in the ballpark of 1/4 wavelength and presented a very high impedance to the Matchbox. Ladder-line feedline lengths for low-impedance antennas need so be close to multiples of 1/2 wavelength, a little over 100 feet on 80m. Unfortunately, a lot of ladder-line-fed 80m dipoles and loops are closer to 1/4WL, around 60 feet or so which may be worst case for limited range tuners like Johnson Matchboxes and internal autotuners. -- 73, Cecil http://www.qsl.net/w5dxp |
how to combat noise?
On 7 Nov 2005 04:04:09 -0800, "andy" wrote:
Here is the situation: have an inverted V for 80m and noise level is S9.. Not good really.... Hi Andrus, Put 1:1 BalUn at end of Coax transmission line as choke. This will isolate local noise from line input. Also, does your transceiver share a circuit (identified at the breaker) with a noisy appliance? Change circuits (go to another outlet that is on a different breaker). Do you have a good RF ground? This goes against much of the common lore of dipoles being independent of ground. True, the antenna alone requires no ground, but somewhere your rig is connected to it, and it can be a very poor connection to a very poor RF ground. This is a very common path for noise to enter your receiver. 73's Richard Clark, KB7QHC |
how to combat noise?
How about trying to locate the source of the noise? Is it in your
house or is it in the neighborhood? It could be something simple like a streetlight buzzing a couple of blocks away. The power company USUALLY pretty good about fixing those kinds of problems. Walk around with a portable AM or shortwave radio and see if you can find the offending pole or whatever. If the noise is in the house, like a dimmer switch, or a tap on lamp, or an appliance, you can fix all that easy enough too. I'm also wondering about your rigs noise blanker. Most modern radios have pretty darn good NB's. Its not a cure, but it should be reasonably effective, unless the noise is a total monster. In that case, all the more reason to find out where its coming from. Good Luck K2TL |
how to combat noise?
wrote:
How about trying to locate the source of the noise? The source of noise at my QTH is the 33 ft. ground wire running down the pole in my front yard from the giant old-as-Methuselah capacitor mounted there. It generates S7 noise on 40m. Maybe an RF choke would work? -- 73, Cecil http://www.qsl.net/w5dxp |
how to combat noise?
Thanks fo sharing. That noise is surely not local: we have powerlines
and industry nearby, LAN in the roofspace, streetlights and so on. I guess that I'll try a full-wave loop directly laying on the roof, feedpoint will be low impedance but how low? 5, 1, 0.5 ohms? Maybe I can transmit to it too if the currents are'nt too high? Is there an antenna design that has high feedpoint impedance to start with, so the impedance would be usable when antenna is layd directly to the roof? Andrus |
how to combat noise?
An outside loop works very well for nulling the noise and still allowing you to hear the weak stations. In most if not all of the ARRL Antenna Books, there is a chapter on a 160 meter receiving loop, you can just scale this back for 80m. I dont have the book here now, I think the lenght was something like 25' or so for 160m. It is made out of a 75 ohm piece of coax that goes around once, connected to a air variable cap. The braid of the coax is cut at a certain point in the loop. Its not that big at all even on 160m and can be put up on a pole and turned by hand or even by a small TV type rotor. On 160m with terrible local power line noise I was able to hear many stations including DX stations on the loop that I could never hear on my Inverted L transmitting antenna on 160m. 73s Craig N0BSA |
how to combat noise?
Here is the situation: have an inverted V for 80m and noise level is
S9.. Not good really.... Dunno...Sounds like it was working to me. I did the next logical step and tryied magnetic loop for 80, reciving only, indoors. What are you trying to hear? Sounds kinda illogical to me, if you are working NVIS... :/ What would you do in this situation? Loop, magnetic loop, dipole, short beverage perhaps??? I'd go back to what was working. The dipole. If the noise is from the shack, the use of baluns or chokes to decouple the feedline can be used. But I think the noise you heard was the normal atmospheric noise you would hear on 80m. I often get S 8-9 noise levels on 75 until winter really kicks in. It's normal. Noise is normal. It's RF. I'd be more worried about *not* hearing any noise, than I would be hearing it. Also, don't lay the antenna on the roof. The higher you can get away from it, the better. A loop will be no better than the dipole in general. Both are about equally efficient if fed with coax. But the loop is more wire, and more work.. :( On 75m NVIS, almost all signals will be over the noise, unless you have a local T storm brewing...Noise is not an issue...Well, not to the ones with decent antennas... To one using a full size dipole fed with coax, even 100w is plenty to get over 95% of the noise you will run into. An S 9 noise level is no big deal if the signals are 20-40 over 9. And this is the norm.. Only the guys running funky gimmick antennas will be hard to hear.. :/ There are some weird ideas around about noise... Almost as many fairy tales as with grounds and shielded loops.. :) No joke. Good antennas pick up noise. That is what they are designed to do. After all, noise is just rf like any other signal. If the antenna is "low" noise, that usually means it's half crippled in some way, unless the noise is ingress from the shack. :/ And thats from the lack of a balun, choke, etc at the feedpoint. Easily cured. Just rolling a few turns of coax into a choke can reduce much of it. But like I say, I think your noise was normal band noise. Listen now, after the cold front blew through the country. I bet it's a tad quieter on that dipole now. MK |
| All times are GMT +1. The time now is 07:42 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com