ATU and Bandpass Filter
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A question about the K2. Taking it to the club for FD and I have a
bandpass filter (CF-20) which says "Never use radio's internal tuner with filter." I have the internal ATU installed. Is there a way to bypass that? Or should I just use the "main antenna" jack (manual says never use if ATU installed) on the bottom left, instead of the two midway in the middle? ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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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.
73 de Matt VK2RQ > On 23 Jun 2017, at 1:13 pm, Lane <[hidden email]> wrote: > > A question about the K2. Taking it to the club for FD and I have a > bandpass filter (CF-20) which says "Never use radio's internal tuner > with filter." > > I have the internal ATU installed. Is there a way to bypass that? Or > should I just use the "main antenna" jack (manual says never use if > ATU installed) on the bottom left, instead of the two midway in the > middle? > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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I think I'm missing something here. I'm not sure I understand putting
the ATU after the filter since the ATU is internal. How does one put it after the external BP filter? On Fri, Jun 23, 2017 at 7:03 PM, Don Wilhelm <[hidden email]> wrote: > 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:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Lane,
Yes, you cannot do that. We are only discussing the need for the filter to have a 50 ohm non-reactive load. So if you are using a bandpass filter designed for a 50 ohm load (most are), then you need to use resonant antennas. What I was saying is that slight variations from a 50 ohm load may be OK ("fine tuning" the load), but many so called multiband antennas do not meet that criteria. Try measuring the feedpoint impedance of a G5RV antenna on various bands, and you will see that it varies considerably. 73, Don W3FPR On 6/23/2017 9:37 PM, Lane wrote: > I think I'm missing something here. I'm not sure I understand putting > the ATU after the filter since the ATU is internal. How does one put > it after the external BP filter? > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Don Wilhelm
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" <[hidden email]> To: [hidden email] 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: ATU and Bandpass Filter
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In reply to this post by Don Wilhelm
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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Don Wilhelm
* 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: [hidden email] [mailto:[hidden email]] On Behalf Of Bob McGraw K4TAX Sent: Friday, June 23, 2017 7:47 PM To: [hidden email] 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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