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  AM/FM antenna absorbs a VHF antennas radiated power ?

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Author Topic:   AM/FM antenna absorbs a VHF antennas radiated power ?
Ridge Runner posted 11-10-2008 12:42 PM ET (US)   Profile for Ridge Runner   Send Email to Ridge Runner  
A marine electronics installer made the following comment to me; he said: "The close proximity of an AM/FM antenna to the VHF antenna as mounted on boats hard or T-top allows it to absorbs approximately 30% of the VHF antennas radiated power when in transmit mode. The AM/FM antenna will also distort the radiation pattern because its magnetic properties will interfere with the magnetic waves being emitted from the VHF antenna." I was wondering if there is any truth in this.
jimh posted 11-10-2008 01:32 PM ET (US)     Profile for jimh  Send Email to jimh     
The statements contain some elements of the real situation. But they're in a jumble of misstatements.

Any conducting object near an antenna affects the antenna, but it is unlikely that you can make any sort of blanket statement that a particular type of antenna will absorb power at a particular percentage.

Whether or not a conductor near a transmitting antenna absorbs power from the transmitting antenna depends on its electrical characteristics. When current is induced into a conductor by the driven element of an antenna, that conductor becomes a parasitic element. Any current flowing in a parasitic element affects the radiation pattern. The entire basis for antenna gain comes from the concept of a mutual impedance between elements of the antenna. So parasitic current flowing in another conductor near the driven element could actually enhance the overall gain of the antenna.

It is entirely possible for energy to be taken from the radiated power of an antenna. When broadcast station WLW in Cincinnati was running 500,000-watts there were stories that local farmers were lighting their homes from the radiated power.

In the case of an antenna tuned to be resonant in the AM Broadcast band, 0.5 to 1.5-MHz, it is difficult to predict its impedance in the VHF Marine Radio band, 156-MHz, as the difference in frequency is in a ratio of 100:1.

jimh posted 11-10-2008 01:51 PM ET (US)     Profile for jimh  Send Email to jimh     
By the way, 30-percent of 25-watts is 7.5-watts. If the other antenna actually absorbed 7.5-watts it would start to warm up. If it delivered the 7.5-watts to the connected receiver, a considerable voltage would be developed. Putting 7.5-watts into a receiver front end will blow most of them to dark black carbon deposits.
Ridge Runner posted 11-10-2008 05:06 PM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
jimh, thank you for the reply. What happens when radar energy comes in contact with either antenna?
WhalerAce posted 11-10-2008 05:49 PM ET (US)     Profile for WhalerAce  Send Email to WhalerAce     
Assuming that the radar energy comes from far-field, you will probably hear the PRI (pulse repetition interval) Brrraaap in your front-end and into the audio section.

Far-field, no problem.

Near-field, smoked front end, just like Jim said

jimh posted 11-10-2008 11:24 PM ET (US)     Profile for jimh  Send Email to jimh     
Assuming a 50-ohm input impedance, if 7.5-watts of power is applied to a receiver input it will produce about 19-volts of signal. That will be enough to blow out most receiver front ends.

Thus if your 25-watt transmitter were actually putting 30-percent of its power into the front end of your broadcast band receiver, it would most likely cause significant damage to the receiver.

jimh posted 11-11-2008 12:01 AM ET (US)     Profile for jimh  Send Email to jimh     
Antennas are usually made of copper or silver which are non-magnetic. Radio waves can be affected by magnetic fields, but it is highly unlikely that a nearby receive-only antenna would posses any sort of permanent magnetic field around it that would affect radio frequency waves passing it. Radio waves which are deflected or focused by magnetic fields usually have those fields created by the flow of strong currents supplied by other means than the radio wave itself.

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