Thursday, November 23, 2006

Does the SID antena need an impedance adapter

The SID antena, doesn't it need some kind of impedance adapter? Just a thought! The feeder will be a 75 ohm coaxial cable.


Blogger Ray Mitchell said...

The SID antenna is passive and a receiver only. Impedance matching is most important in radio transmitters. We designed the antenna input system not to care about such things as impedance, wavelength, resistance, etc. Go ahead and build your antenna and hook it up, it will work as advertised!

For those interested… An aerial used for transmitting RF that is not impedance matched would reflect the energy back from the antenna causing standing waves on the line; it makes the system inefficient. And another consequence of mismatching the transmission line impedance is that the line at the transmitter end would appearing to have either inductive or capacitive reactance that would detune the inductance-capacitance (LC) circuit to which it is coupled; and usually that circuit is the final stage of the RF amplifier… and yes, that would alter the expected frequency you were trying to transmit on.

Again, this is not a concern for the SID monitor antenna. Of course you may find that you have other problems of finding a location, orientation, adjusting the gain, etc. and think that it something to do with the antenna itself. Sometimes the day chosen to set the antenna up coincides with maintenance day at the transmitter site… These issues are described in our manual.

Good luck with the installation.

12:39 PM  
Anonymous Anonymous said...

Since this site isn't using Blogger Beta yet, I'm unable to post here under an original title. Blogger is not accepting any new accounts using the original Blogger site.

I just finished setting up my VLF antenna, setting a suitable location, connecting all the necessary cables and installing / configuring the SIDMON software. By all accounts, I have a strong signal (based on the audio output from the monitor). Either I'm really missing something (and I don't think that I am) or there is something *really* wrong with the SIDMON VLF monitor or DATAQ module. I am unable to calibrate the SIDMON VLF monitor based on the directions specified in the manual. According to those instructions, the voltage must be set between -1.25 and -1.50 at midday or at other values based on the time of day. Turning the RF Gain adjustment screw as directed in the manual has *no* effect on the indicated voltage. Now, I'm assuming that this voltage reading is indicated under the 'Data' heading. In addition, the antenna has no effect on this. With our without the antenna connected, this value *always* remains the same. Needless to say, I am unable to collect any data aside from a flat line at 4.98 volts.

Any assistance at this juncture would be a great help!

Tom Madigan
Member, Adjunct Faculty, Suffolk County Community College

3:59 PM  

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