Re: Topband: Feedline Choke Placement in RX systems
In the case of the YCCC 9-circle array, which I designed, common mode suppression is deployed at both the antenna ends of the feedlines and at the combiner where the antenna feedlines come together. The YCCC feedpoint preamp already includes common mode suppression, so additional feedline chokes should not be necessary there. The reason for additional common mode suppression at the combiner is as follows. The 9-circle system is different than other receiving arrays in that there is a center element which is usually collocated with the phase combiner because that is a convenient place to install the combiner. However, the convergence of all the feedlines at the combiner, very close to the center element, can create severe feedline-induced pattern distortion unless common mode suppression is deployed at the ends of each of the feedlines where they enter the combiner. This is because the outsides of the coax feedline shields form what looks like a field of wires that intersect near the center of the array, much like a radial system. The pattern distortion is created by common mode currents on the feedlines coupling to the center element, even though there is no direct electrical connection to the center element. I never gave consideration to this effect until I deployed my first prototype system in the field and discovered the pattern of the array was not very good despite VNA measurements on the combiner and preamps that showed accurate gain and phase. I eventually discovered through antenna modelling that strong coupling exists between the feedline shields and the center element. The addition of the feedline chokes at the combiner effectively breaks the electrical path for current flow on the outside of the shields of the feedlines where they intersect at the combiner. This almost completely eliminates the pattern distortion. Once I deployed the feedline chokes, the pattern cleaned up dramatically, just as the modelling predicted. The YCCC combiner does not incorporate the common mode suppression internally. This is best done on the feedlines themselves, installing feedline chokes a few feet away from the combiner. 73, John W1FV From: Dennis W0JX [mailto:w...@yahoo.com] Sent: Tuesday, November 30, 2021 12:48 PM To: topband@contesting.com; John Kaufmann Subject: Feedline Choke Placement in RX systems K9YC made an interesting comment in his post, saying that the feedline chokes would be most effective if placed at the antenna end of the feedline. W1FV's 9 circle YCCC array has them right at the controller. The older K7TJR combiner utilized no feedline chokes. However, I believe that Lee's new design has built in common mode protection on the combiner circuit board. So what is the best way? Is placement dependent upon the combiner design? BTW, K3LR is using 2.4 in #31 toroids with at least 24 turns of RG-179 as feedline chokes in his 8 circle HiZ array and I believe that there are chokes on both ends of the antenna feedlines. Of course, K3LR operates in a high intensity, multi-transmitter environment and may need all that choking. 73 Dennis W0JX _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Feedline Choke Placement in RX systems
I do have very high CMI chokes (0ver 10K ohms) at the feedpoint of each vertical element and at the HI-Z combiner box for each element. This may seem extreme, but I am more than pleased with the results. I have been working on improving the low band RX systems here for many years. I have a 113 ft diameter HI-Z 8 circle. And a 113 ft diagonal HI-Z 4 square. The two arrays have identical element configurations - it is just one is 8 and the other 4. The array centers are spaced 500 feet apart and they are broadside to Europe. The loss and electrical length of the feed lines going to the two arrays are calibrated so they are the same. We have a custom switch that allows us to listen to the 8 circle alone, the 4 square alone or put them in phase or out of phase. Both arrays use 23 foot vertical elements (DXE AL-24) with two 4 ft ground rods at the base. I am using older HI-Z element buffers that have been modified to take power directly - not via the RG6 bias tee coax. Any amount of moisture in a system that feeds DC and RX RF together can have noise problems. I am confident in my ability to weatherproof, however I wanted the best possible result. So there are high CMI chokes in the RF and DC lines - 24 chokes and 24 feedlines (one for RF and one for DC). There are so many things in the arrays that we cannot control, I try to control as many items as I can. I built all new verticals (12) this summer and used weatherproofing at each element joint to keep any moisture out of the element junctions. I also replaced the older dual shield RG6 with the newer quad shield RG6. Lastly, I changed out my older HI-Z combiners for the latest ones that Lee makes. Surface mount components, many with 1% tolerances and improved F connector mountings. W8WWV developed a choke system that we use at the element feed - that makes these arrays deaf above 4 MHz. That helps keep down the possible mixing with all of the multi multi TX RF here. So - how do the HI-Z arrays work? Very well. There is great pattern "snap" as you rotate the array around, with close to 30 dB of F/B. John, N2NC was the 160 SSB operator here for CQWW and he told me the arrays are working very well. I was the 160 meter operator this past weekend and even under poor conditions I thought the performance was very good. Sunday morning I worked ZM4T who was not very strong, at 1115 Z I worked JW7QIA, then an amazing QSO at 1130 Z with RW0A in Zone 18. All using the HI-Z arrays. The ability to feed these arrays out of phase is impressive as it sometimes fills in - where a station is not directly at 45 degrees, 135 degrees etc. The best position for RW0A was 45 degrees - both arrays but fed out of phase. That put a strong lobe at 15 degrees - perfect! I have worked Zone 18 before on 160, but it was always done in the evening here - around 0130 Z. No more Beverages at K3LR... 73 Tim K3LR -Original Message- From: Topband [mailto:topband-bounces+k3lr=k3lr@contesting.com] On Behalf Of Lee STRAHAN Sent: Tuesday, November 30, 2021 2:35 PM To: Dennis W0JX; topband@contesting.com Subject: Re: Topband: Feedline Choke Placement in RX systems Greetings All, For the Hi-Z arrays any additional common mode choking for most all installations would not be needed. You are correct at K3LR there are added common mode chokes in the element feedlines. I am fairly certain that adding these helped more with interstation isolation. I have not heard for certain that K3LR saw any noise reduction in that system due to the addition of chokes. He has an 8 element version that already included combiner input common mode isolation. Tim may want to comment on this. All my Hi-Z amps have had common mode isolation in them since day one. So placing another choke at the Hi-Z amps seems to be not necessary. In the older designs there was no common mode isolation on the combiner inputs on the smaller arrays meaning the three element and the 4 element. The 8 element arrays were designed with common mode isolation at the combiner inputs making these systems completely isolated. In the new redesigns I have added common mode isolation to the inputs of the new 4 e lement array design just because I thought it certainly won't hurt anything but possibly could help. The Three element array has so much less combiner loss and its element feedlines are so short I have not added any common mode isolation to its inputs. I have not seen any need here in many tests of that array. My take on the placement would be to put them closest to the controller for the home brew W1FV array. From my older W1FV schematics his high impedance amps have common mode isolation built in so placing added isolation would be better at the combiner. Building your own high impedance amps would certainly need common mode isolation. You are correct Dennis as it depends on the overall system design. Lee K7TJR Hi-Z Antennas -Original Message- From: Topband On Behalf Of Dennis W0JX via To
Re: Topband: Feedline Choke Placement in RX systems
On 11/30/2021 11:34 AM, Lee STRAHAN wrote: All my Hi-Z amps have had common mode isolation in them since day one. So placing another choke at the Hi-Z amps seems to be not necessary. Makes sense to me! Thanks Lee. 73, Jim K9YC _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Feedline Choke Placement in RX systems
Greetings All, For the Hi-Z arrays any additional common mode choking for most all installations would not be needed. You are correct at K3LR there are added common mode chokes in the element feedlines. I am fairly certain that adding these helped more with interstation isolation. I have not heard for certain that K3LR saw any noise reduction in that system due to the addition of chokes. He has an 8 element version that already included combiner input common mode isolation. Tim may want to comment on this. All my Hi-Z amps have had common mode isolation in them since day one. So placing another choke at the Hi-Z amps seems to be not necessary. In the older designs there was no common mode isolation on the combiner inputs on the smaller arrays meaning the three element and the 4 element. The 8 element arrays were designed with common mode isolation at the combiner inputs making these systems completely isolated. In the new redesigns I have added common mode isolation to the inputs of the new 4 e lement array design just because I thought it certainly won't hurt anything but possibly could help. The Three element array has so much less combiner loss and its element feedlines are so short I have not added any common mode isolation to its inputs. I have not seen any need here in many tests of that array. My take on the placement would be to put them closest to the controller for the home brew W1FV array. From my older W1FV schematics his high impedance amps have common mode isolation built in so placing added isolation would be better at the combiner. Building your own high impedance amps would certainly need common mode isolation. You are correct Dennis as it depends on the overall system design. Lee K7TJR Hi-Z Antennas -Original Message- From: Topband On Behalf Of Dennis W0JX via Topband Sent: Tuesday, November 30, 2021 9:48 AM To: topband@contesting.com; John Kaufmann Subject: Topband: Feedline Choke Placement in RX systems K9YC made an interesting comment in his post, saying that the feedline chokes would be most effective if placed at the antenna end of the feedline. W1FV's 9 circle YCCC array has them right at the controller. The older K7TJR combiner utilized no feedline chokes. However, I believe that Lee's new design has built in common mode protection on the combiner circuit board. So what is the best way? Is placement dependent upon the combiner design? BTW, K3LR is using 2.4 in #31 toroids with at least 24 turns of RG-179 as feedline chokes in his 8 circle HiZ array and I believe that there are chokes on both ends of the antenna feedlines. Of course, K3LR operates in a high intensity, multi-transmitter environment and may need all that choking. 73 Dennis W0JX _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Feedline Choke Placement in RX systems
The feedline (if coax) outer braid responsible for imbalance RF radiation, is fed from the antenna end. Choking it there eliminates a RF active length. vk4tux On 1/12/21 03:48, Dennis W0JX via Topband wrote: K9YC made an interesting comment in his post, saying that the feedline chokes would be most effective if placed at the antenna end of the feedline. W1FV's 9 circle YCCC array has them right at the controller. The older K7TJR combiner utilized no feedline chokes. However, I believe that Lee's new design has built in common mode protection on the combiner circuit board. So what is the best way? Is placement dependent upon the combiner design? BTW, K3LR is using 2.4 in #31 toroids with at least 24 turns of RG-179 as feedline chokes in his 8 circle HiZ array and I believe that there are chokes on both ends of the antenna feedlines. Of course, K3LR operates in a high intensity, multi-transmitter environment and may need all that choking. 73 Dennis W0JX _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
