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CB antennas and neighbors
Which omnidirectional base antenna will put out the least TV and telephone interference in the neighborhood? |
"Glenn S." wrote in message ... Which omnidirectional base antenna will put out the least TV and telephone interference in the neighborhood? It doesn't depend on the antenna almost at all. The main cause of interference is from three sources. One a dirty transmitter, the second is from receiver front end over load, and the third is from RF getting in to the electronics directly. The first one you can do something about by using a clean transmitter. In other words one that hasn't been messed around with such as peaking etc. Peaking a CB radio requires messing around with the tuned output circuits which are responsible for presenting the proper load to the output transistor. These circuits are also responsible for filtering out the harmonic signals that cause interference to TV signals. The people who peak radios rarely if ever check the radio output for spectral purity and the now peaked radio may have a much dirtier output that it had before it was touched. Receiver front end over load you can do little to prevent except by keeping your antenna as far away from your neighbor's TV/radio antenna as you can get. As far as RF getting in to the electronics directly you can suggest they try using some RFI suppression chokes they can buy at the local Radio Shack. Other than that the fault lies with the manufacture of the malfunctioning electronic device. The use of a amplifier is not recommended since one it isn't legal to use on the CB band, and second it makes all of the above problems much worse. As long as you operate using legal and unmodified equipment you will be in the clear even if one or more of your neighbors complains to the FCC. The was a case here in the Detroit area 3 to 4 years ago where a local CBer was coming in over his neighbor's TV's, radios and cordless phones. The FCC investigated his station and found everything in order, no illegal or modified equipment. The FCC told the city there was nothing they could do since his station meet all FCC specifications and suggested the people having problems either complain to the manufacture of the affected device, or buy some RFI suppression devices and try them out. -- Leland C. Scott KC8LDO Wireless Network Mobile computing on the go brought to you by Micro$oft |
On Mon, 20 Sep 2004 00:33:28 -0400, "Leland C. Scott"
wrote: "Glenn S." wrote in message .. . Which omnidirectional base antenna will put out the least TV and telephone interference in the neighborhood? It doesn't depend on the antenna almost at all. The main cause of interference is from three sources. One a dirty transmitter, the second is from receiver front end over load, and the third is from RF getting in to the electronics directly. That's not entirely true. The antenna DOES have a part to play in the whole mess. Some of the "stick"-type antennas, such as the A-99, have poor decoupling and this allows for significant coaxial shield radiation. Some of these antennas also concentrate a good portion of their near field radiation in places where it would exacerbate front end overload or couple R.F. into house wiring. An otherwise clean transmitter coupled to an antenna with the above characteristics can cross the line between no RFI and significant RFI. In some cases, this can be mitigated somewhat by moving the antenna (usually raising it) to another area, where it's radiation will not couple as much R.F. into neighboring premises. A better solution would be to run an antenna with ground plane radials, such as a Sigma 5/8 wave or similar. Dave "Sandbagger" http://home.ptd.net/~n3cvj |
An otherwise clean transmitter coupled to an antenna with the above
characteristics can cross the line between no RFI and significant RFI. In some cases, this can be mitigated somewhat by moving the antenna (usually raising it) to another area, where it's radiation will not couple as much R.F. into neighboring premises. A better solution would be to run an antenna with ground plane radials, such as a Sigma 5/8 wave or similar. Dave "Sandbagger" http://home.ptd.net/~n3cvj _ And the running of certain amplifiers will NOT compound or add additional RFI in such cases. A perfect linear for cbers is a palomar, as it is not class C, which certain long-time hammie posters in rec.radio.cb consider to be some kind of devil-spawned conspiratorial gadget to reek havoc and cause misery in their radio lives. |
Which omnidirectional base antenna will put out the least TV
and telephone interference in the neighborhood? A resonant one, properly decoupled from the feedline. Someone with an antenna analyzer can check this for you, you're not just looking for low SWR, you're looking for little or no reactive component to the impedance in that band. As an extreme bad example, I saw a radio shack salesman explain to a customer how he could use their discone antenna (the one without the top element!) for CB. That antenna is dramatically too small to be resonant on 11m, it would have just been an efficient radiator for any VHF-UHF band harmonics and noise that the rig might produce. -- KC6ETE Dave's Engineering Page, www.dvanhorn.org Microcontroller Consultant, specializing in Atmel AVR |
"Dave Hall" wrote in message ... On Mon, 20 Sep 2004 00:33:28 -0400, "Leland C. Scott" wrote: "Glenn S." wrote in message .. . Which omnidirectional base antenna will put out the least TV and telephone interference in the neighborhood? It doesn't depend on the antenna almost at all. The main cause of interference is from three sources. One a dirty transmitter, the second is from receiver front end over load, and the third is from RF getting in to the electronics directly. That's not entirely true. The antenna DOES have a part to play in the whole mess. Some of the "stick"-type antennas, such as the A-99, have poor decoupling and this allows for significant coaxial shield radiation. Some of these antennas also concentrate a good portion of their near field radiation in places where it would exacerbate front end overload or couple R.F. into house wiring. An otherwise clean transmitter coupled to an antenna with the above characteristics can cross the line between no RFI and significant RFI. In some cases, this can be mitigated somewhat by moving the antenna (usually raising it) to another area, where it's radiation will not couple as much R.F. into neighboring premises. A better solution would be to run an antenna with ground plane radials, such as a Sigma 5/8 wave or similar. Dave "Sandbagger" http://home.ptd.net/~n3cvj That would be true of course for his own situation, the classic RF in the shack problem, but he specifcaly asked about interference to the local neighborhood if you re-read his post. In that case I doubt the antenna type will have any significant impact. -- Leland C. Scott KC8LDO Wireless Network Mobile computing on the go brought to you by Micro$oft |
"Twistedhed" wrote in message ... An otherwise clean transmitter coupled to an antenna with the above characteristics can cross the line between no RFI and significant RFI. In some cases, this can be mitigated somewhat by moving the antenna (usually raising it) to another area, where it's radiation will not couple as much R.F. into neighboring premises. A better solution would be to run an antenna with ground plane radials, such as a Sigma 5/8 wave or similar. Dave "Sandbagger" http://home.ptd.net/~n3cvj _ And the running of certain amplifiers will NOT compound or add additional RFI in such cases. A perfect linear for cbers is a palomar, as it is not class C, which certain long-time hammie posters in rec.radio.cb consider to be some kind of devil-spawned conspiratorial gadget to reek havoc and cause misery in their radio lives. Twist the amp doesn't have to be class "C" to cause problems. Even a class "B" linear amp is not completely clean. In particular solid state amps are a problem because many are push-pull designs with no filtering on the output. A push-pull design has low levels of even harmonic output since the even order signals cancel out in the output transformer. And that is only for "well matched" power transistors. The problem with the push-pull design is the odd order harmonics are not attenuated at all! If you care to look at Ham rigs that use wide band solid state push-pull amplifier output for HF operation you will see band pass filters on the output. In fact these filters are switched in and out depending on the band in use. That's how the manufactures keep the output clean enough to meet FCC specifications. Tube amplifiers are another story. These amplifiers can be relativity clean IF the output is designed correctly. The critical section is the output matching circuit. This circuit performs two important functions; load matching and harmonic filtering. Some of the posters on this board in the past don't seem to grasp these two ideas well. They seem to think that if the amp loads up OK then all is well - not true. If you go through the math you will discover that there is no unique solution for the value of the matching section component values. So how do the designers pick the values they do? They pick an operation "Q" for the matching section, that forces a particular value for the components. The significance of the "Q" value is it is a measure of how frequency selective the matching section becomes. The higher the "Q" the sharper the response becomes, thus the less harmonic energy is coupled to the output. Pick a "Q" too low and you have significant harmonic output from the amplifier. Pick a value too high makes the amplifier a pain to use, in other words the amplifier has to be retuned for small shifts in operating frequency. From either experience, or through detailed and complex calculations, acceptable values for "Q" have been determined that would yield a tube amplifier matching circuit that is likely to have acceptably low levels of harmonic output content. If somebody says their amp is "clean" well the only sure way to know is to test it with a spectrum analyzer. If you look at reviews for Ham rigs, and power amplifiers in particular, you will see spectrum analyzer screen shots so the buyers can see for themselves just how clean the output happens to be. And I haven't seen a spectrum analyzer screen shot for a 11m solid state amplifier yet. If you know of any it would be interesting to see them. -- Leland C. Scott KC8LDO Wireless Network Mobile computing on the go brought to you by Micro$oft |
"I Am Not George" wrote in message m... (Twistedhed) wrote in message ... An otherwise clean transmitter coupled to an antenna with the above characteristics can cross the line between no RFI and significant RFI. In some cases, this can be mitigated somewhat by moving the antenna (usually raising it) to another area, where it's radiation will not couple as much R.F. into neighboring premises. A better solution would be to run an antenna with ground plane radials, such as a Sigma 5/8 wave or similar. Dave "Sandbagger" http://home.ptd.net/~n3cvj _ And the running of certain amplifiers will NOT compound or add additional RFI in such cases. A perfect linear for cbers is a palomar, there you go again encouraging lawbreaking as it is not class C, which certain long-time hammie posters in rec.radio.cb consider to be some kind of devil-spawned conspiratorial gadget to reek havoc and cause misery in their radio lives. LOL its 'wreak' havoc not reek, journalism boy Are you sure? |
That would be true of course for his own situation, the classic RF in the
shack problem, but he specifcaly asked about interference to the local neighborhood if you re-read his post. In that case I doubt the antenna type will have any significant impact. Rf on the feedline will be radiated through the whole path, and probably much lower and closer to the neighbor. RF on the feedline is never a good thing. -- KC6ETE Dave's Engineering Page, www.dvanhorn.org Microcontroller Consultant, specializing in Atmel AVR |
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