- January 18, 2007 at 2:56 pm #6809wdcxParticipant
Total posts : 451
Since wider bandwidth on AM antennas results in better audio performance, how does one calculate the BW of say a CB whip compared to a 1/2 copper pipe operating at 1610 KHZ?January 18, 2007 at 3:58 pm #14655RichGuest
Total posts : 45366
The r-f bandwidth will depend on the diameter of the radiator as well as the operating frequency and the other r-f resistances in your system. The biggest resistance contributor most likely is the r-f ground resistance, and that is almost certainly unknown.
About all that can be said here is that when other things are equal, the SWR bandwidth of the system will be greater as radiator OD increases.
//January 19, 2007 at 4:48 am #14658classEradioGuest
Total posts : 45366
As pointed out above, the bandwidth of an antenna is based on many factors. The diameter of the radiator, as a portion of a wavelength is, in this case – as comparing a CB whip to a copper pipe, is probably negligible.
Interestingly enough, I have measured the common point impedance of many commercial broadcast stations, back when I was in commercial broadcast engineering. Many, and in particular, directional stations, have a fairly narrow “bandwidth”, with respect to the common point impedance. I’ve seen some, where, 10kHz out from the center frequency, the impedance was off by 20 or 30 %.
Some ways to check this:
If you have a known, flat response transmitter, you could sweep the audio from, say 20hZ to 10kHz and observe changes in the modulation percentage. This may speak to an impedance change in the antenna. Note: if you are not sure about the frequency response of your transmitter / audio system, this test will not be meaningful.
You can characterize your transmitter / audio system frequency response by operating the transmitter into a purely resistive load (a resistor of the proper value to present a valid load to the transmitter), and then, using a good audio oscillator, sweep the audio frequency range and observe the modulation percentage on a modulation monitor or other measurement device – one that will allow you to see small changes in modulation precentage, and hopefully a VU indication. This should actually be done in any event, just so you know what the system is doing.
A good off-air monitor, receiving the actual signal as transmitted, will be of tremendous value in ensuring your system is flat, at least within the usable audio range.
You can also listen, using a fairly selective receiver – preferably one with a signal strength meter – to the relative sideband power of a 10kHz modulated sine wave. There will be one sideband, 10kHz from the carrier (on each side). With a pure 10kHz modulated sine wave, the sidebands will, in fact, appear as “carriers”, and can therefore be tuned on a selective receiver as an independent signal. If one sideband is significantly stronger than the other at 10kHz, your antenna system / transmitter output network is probably fairly sharp. In many cases, the impedance will be higher on one side of resonance than the other, and the sideband power is rarely equal under these circumstances. If the output network is very sharp, you could experience sideband attenuation, and the atteunuation may be more or less equal for both sidebands. The first test is more valid in this case.
Sidebands are a result of the modulation process, and this is where the information (modulation) is carried. When you modulate an AM transmitter, sidebands are produced above and below the carrier frequency, equal to the carrier frequency plus and minus the modulating frequency. So, if you modulate your transmitter with a 10kHz pure sine wave, you will produce sidebands at plus and minus 10kHz from your carrier (center) frequency. The bandwidth of the signal is defined as the amount of spectrum consumed by the entire signal, including the sidebands. The exact measurement must specify how many dB (or actual power) that is present at or outside of the defined bandwidth. For commercial broadcast stations, this specific value is part of the FCC rules governing proper station operation.
Hope this helps !January 19, 2007 at 11:59 am #14670wdcxGuest
Total posts : 45366
Good info here. 🙂
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