* Band pass filters are likely to be specified for operation at any impedance consistent with a certain maximum allowable swr. Mine (made by LBS) tolerate swr up to 2:1. As long as the antenna meets that requirement, there is no need for a tuner between filter and antenna.
* A good multiband antenna that offers a 2:1 match or better over multiple frequency ranges of interest will work with filters just as well as a monoband antenna with similar spec's. * There are "multiband" antennas out there that do not by design meet the 2:1 requirement, particularly not on all the bands supposedly covered. The G5RV would be one example. Such antennas owe their popularity to the fact that they provide something resembling resonance on certain bands (but with swr higher than 2:1), and many tuners can cover up the mismatch. For a given multiband antenna it would be a good idea to measure swr across each band before considering use of bandpass filters, and verify swr lower than 2:1. Yes, a tuner could be placed between antenna and filter, but it may not be very practical. The tuner would have to be tuned up on different frequencies as needed, and the measurement should then be done at the radio side of the tuner (not measuring through the filter). * What happens if you use the bandpass filter with an antenna that is not 2:1 swr or better? Isolation is likely to suffer, but whether isolation is good enough will depend on individual requirements. Power handling is a major consideration. I have been advised to substantially oversize the power rating of the filter when connecting to an antenna moderately worse than 2:1. That should be kept in mind when an antenna such as a G5RV is to be used. * A true multiband antenna (i.e. one that simultaneously provides low swr on each band to be used) can be used with a multiplexer that provides separate radio-side connectors for each band. Such a multiplexer is always used with separate bandpass filters on each of the radio-side connectors. In this situation, the multiplexer provides isolation that adds to that provided by the bandpass filters. The radios using the different branches of this arrangement are likely to see isolation much better than they would with separate antennas and bandpass filters only. Multiple radios on different bands can transmit simultaneously through the one shared multiband antenna. The need to orient separate antennas carefully relative to each other is thus eliminated. The cost of the multiplexer can be balanced against the cost and effort for installing separate antennas. Of course, a multiband antenna does not offer the freedom to adjust pointing direction independently for the different bands. To qualify as a multiband antenna in the multiplexed context, it must be simultaneously "resonant" for multiple bands. An antenna that must be retuned when changing bands, such as with a remote motor-driven adjustement, no matter how quick and automated, would not work. * The multiplexer is similar to the bandpass filters in requiring low swr (such as 2:1 or better), so if modestly higher swr's will be encountered, both the multiplexer and bandpass filters must be substantially oversized in terms of power rating. 73, Erik K7TV -----Original Message----- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Bob McGraw K4TAX Sent: Friday, June 23, 2017 7:47 PM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] ATU and Bandpass Filter If one chooses to use band pass filters then one should anticipate using an external antenna tuner as well. This of course depends largely on the antenna and the impedance of the antenna at a given frequency. If one is using resonant antennas with a reasonably close 50 ohm impedance then all is well. If one is using a multiband antenna the impedance is likely to vary widely and be far from 50 ohms. As to a beam, if it is correctly adjusted, it should provide a reasonably close impedance to 50 ohms on all band for which it is designed. I find many hams are grossly over concerned with SWR values. For example; 100 ft of RG-8X operating with a 2:1 SWR at 14 MHz will have a total loss of 1.5 dB. Where as the same length of coax with a 1:1 SWR will have a loss of 1.2 dB. A difference of only 0.3 dB. With the same configuration at 28 MHz, the loss with a 2:1 SWR will be 2.1 dB and with a 1:1 SWR the loss will be 1.8 dB a difference of 0.3 dB. My point is..........hams are grossly over concerned about having a 1:1 match. Factually, it isn't that dang important. As you can see, there is little difference in total loss. Thus of 100 watts power fed into the line at 14 MHz and a 2:1 SWR the power at the antenna will be 70 watts. Where as 100 watts fed into the line at 14 MHz with a 1:1 SWR the power at the antenna is 75.8 watts. These are real numbers, thus a difference of only ~5 watts. The only thing affecting loss is the loss in the transmission line which is present to some degree regardless of the SWR match. 73 Bob, K4TAX ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to ebasil...@cox.net ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
