Hi Don,
The technique you use with monoband horizontal dipoles and bandpass filters is exactly what we do at the W3AO Field Day site, except we use monoband Yagis rather than dipoles on 40, 20, 15 and 10 meters. (we also use dipoles on 40M). The recent recommendation in the Elecraft Newsletter about using two perpendicular horizontal dipoles isn't very effective, and the concept was incompletely presented. In order to achieve excellent isolation between two perpendicular dipoles, two conditions must be met: - the second dipole needs to be almost exactly perpendicular to the first dipole, even a five degree error significantly reduces the isolation. - most importantly, the second dipole must be perpendicular to the center of the first dipole. An offset of just a few feet left or right of center significantly reduces the isolation. The advantage of this technique is that two horizontal dipoles can be installed in a physically small space with very high isolation, but the big disadvantages are that the perpendicular dipoles must be precisely positioned a nd inevitably at least one of the dipoles is likely to be oriented to an non-optimum azimuth, We're fortunate at W3AO to have a 1000 x 200 foot open grass field for our Field Days, The technique we use with excellent results is to place our antennas for the same band end-to-end with 300 feet of separation between adjacent antennas. While the isolation is significantly less than two precisely positioned perpendicular horizontal dipoles, it has the big advantage of having both antennas oriented to the same azimuth. With four 20 meter Yagis sited end-to-end with 300 foot separation between adjacent antennas we routinely operate four transmitters (CW SSB RTTY and GOTA) on 20 meters with no trace on interference. 73 Frank W3LPL ----- Original Message ----- From: "Don Wilhelm" <[email protected]> To: [email protected] Sent: Saturday, June 24, 2017 12:03:29 AM Subject: Re: [Elecraft] ATU and Bandpass Filter What Matt said is true. However, if your antennas are close to resonance, go ahead and use the ATU to "touch up" the tuning. If you are trying to use a multiband antenna such as a G5RV and such, you might as well forget the advantage of the bandpass filter. Our local club discovered that multiband antennas were a major problem at multi-transmitter Field Day sites. We now use single band dipoles, and yes we use a bandpass filter for each of the FD bands. 73, Don W3FPR On 6/23/2017 4:35 AM, Matt Maguire wrote: > The problem is that the filter is designed to work with a 50 ohm > characteristic impedance (ie. with a 1:1 VSWR). This means you need to put > ATU *after* the filter, not before, otherwise the filter will not work > properly. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
