As Brian points out, simple paralleling via a "T" connector is likely to produce strange results depending on types of antennas used and feed-line lengths. Best to use some sort of combiner or multiplexer that is indifferent to competing other-band impedances.
Another consideration is receiving antenna pattern and the resulting noise rejection (or not). If one were to try to monitor beacons in at a particular heading while search for a rare one, the antenna pattern to peak up the signals and suppress unwanted noise and QRM becomes important. If a two antenna "T"-connector approach is used, the "wrong-band" antenna will contribute noises from weird directions that compromise the "correct" antenna's receive performance. A bandpass style multiplexer would keep noise/QRM contributed by the "wrong-antenna's" goofy, off-frequency directivity pattern from degrading the S/N (S/QRM) performance of the "right" antenna. I'm generally happy with the directivity of my M2 7&10-30LP8 log periodic on most bands of interest, so I won't need a band-pass multiplexer to get decent noise/QRM performance for simultaneous multi-band receiving. Cheers, Mike -W8MM -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Brian Lloyd Sent: Wednesday, August 01, 2012 1:55 AM To: George Allen Cc: [email protected] Subject: Re: [FlexEdge] The Flex Insider - Issue 2...re antennas a thought On Tue, Jul 31, 2012 at 7:54 PM, George Allen <[email protected]>wrote: > Gerald provides a very interesting discussion of antennas in the > FlexInsider Issue 2; but, what is wrong with just paralleling antennas for > different bands together? > > > > Lets suppose that we wish to monitor both 20 meters and 80 meters at the > same time. If we have two antennas that are resonant respectively on 20 > and 80 meters and we parallel them together, the receiving SCU's should be > happy. Yes, even though there is a high impedance for the out-of-resonant > antenna, the receive losses will be low and we will get signals from both > antennas. But, nothing bad should happen. > Why would you assume that the antenna operating out of its normal operating frequency is high impedance? Remember that the feedline operates as a transformer and might transform a high-impedance at the antenna into a very low impedance at the feed-point. What you are suggesting might work in some instances but it by no means is guaranteed to work in another. No, if you want multi-band performance you want to use a multi-band antenna or you want to use a broad-band antenna like a log-periodic or a T2FD. (http://en.wikipedia.org/wiki/T2FD_Antenna) -- Brian Lloyd, WB6RQN/J79BPL _______________________________________________ Flexedge mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for posting topics related to SDR software development and experimentalist who are using beta versions of the software.
