Subj: CATV Hardline -> TH6 Summary Date: Thu, May 18, 1995 8:22 PM CDT From: cgate!ocker@zool.interaccess.com X-From: cgate!ocker@zool.interaccess.com (Charlie Ocker) Sender: owner-cq-contest@tgv.com Reply-to: cgate!ocker@zool.interaccess.com (Charlie Ocker) To: cq-contest@tgv.com Hola, fellow contesters! The following messages were received in response to my query about using 75 ohm CATV Hardline with a TH6 tribander. Good luck with your spring/summer antenna projects! Vy 73, Charlie KD5PJ/9 ocker@chasind.com P.S. I originally sent this summary only to those that requested it to con- serve net bandwidth, but have been getting even more requests for it, so I have decided to post the summary for all. P.P.S Thanks also to K8MR whose response is also included. ------------------------------------------------------------------------------- Personally, I wouldn't worry about the mismatch. Years ago we built matching transformers for KM1C's 10 meter stack. Even with the 200+ feet of hardline, the mismatch loss would have been under 1/2 dB without the transformers. And this was on 10 meters, so other bands would have been less. If you can make the feedline 1/2 wavelength at 40 meters (or multiple thereof), then it will mirror the antenna impedance at the other end, for all bands 20-10. Then the impedence of the feedline becomes immaterial. 73 - Jim AD1C ------------------------------------------------------------------------------- The method I have used very successfully feeding both mono-band and multi-band antennas is as follows: cut the catv to multiples of an ELECTRICAL 1/2 wavelength at the lowest frequency of operation. For a TH6/7 use 40m as the starting point...this gives you 3/2 waves on 15. Of course the antenna doesn't work on 40. If you cut for 1/2 electrical wavelength on 40 that gives you 2/2 wavelengths on 20, 3/2 on 15, and 4/2 on 10. Why 1/2 electrical wavelength? The antenna impedance is reflected down the line @ every 1/2 wavelength. To come up with the electrical 1/2 wavelength use the 'real' 1/2 wavelength and multiply by the velocity factor of the CATV; typically on the order of .80-.83 for most 75ohm catv. RG-8 is .66. What this means is the cable will be slightly shorter per 1/2wave than a dipole. We have literally thousands of feet of this stuff in use at the NB1H/NX1H/K1TR/K1RX actually K1MNS Multi-Multi. I'm sure you have also discovered that 3/4" catv is real easy to adapt with the hacksaw method and a double female barrel connector. Weather proof liberally and it will last a long time. 73, Marty, WT1S/0 -------------------------------------------------------------------------------- >3) just feeding it and not worry about mismatch Go for it! Remember you can always adjust the SWR at the xmtr by changing the overall length of cable. I have jumpers made up, and by serendipity in most cases, I find one that will lower the SWR sufficiently to avoid problems in the shack, like the blinking of the monitor. Take a look at TL.EXE that came bundled with the latest Antenna Book, or download it from the ARRL BBS. You'll see that the mismatch (assuming the antenna is 50 ohms) causes negligible (like .08dB) loss. I often wonder if using an 1/4 wave xfrmr, which implies using one on each end of the hardline and its 4 connectors, actually cause more loss than living with the mismatch. ....................................................................... 73 de Walt Kornienko <<< K2WK >>> Internet: waltk@pica.army.mil DX PacketCluster: K2WK > W3MM (FRC) Packet: K2WK@N2ERH.NJ.USA.NOAM Member: The Frankford Radio Club #1 Contest Club In The Universe -------------------------------------------------------------------------------- Number 3: Just feed it. Tree *** and more which came later *** At N6TR, most all of my antennas are monobanders, so I have chosen to make the simple coax impedance matching transformmers as shown in the NCJ. However, these devices are not so simple if you are using a tri-bander. There is one thing you can do. There is some magic length (around 140 feet or multiples of that) where the coax is a half wavelength on all three bands. This means the impedance of the antenna will be what the impedance is at the transmitter. You could put some 50 Ohm coax on one, or both ends, to make it long enough (you will need some on the antenna side for rotating). While you will still have a 1.5 to one SWR inside the coax, since it is low loss, it shouldn't cost you anything. And then your wattmeter shows a great match and everything is fine. But, this is more for show than anything else. Unless you are just using a barefoot transistor rig without an antenna tuner, having a SWR of 1.5 to 1, or even a little higher, shouldn't be a problem to match to. Tree N6TR -------------------------------------------------------------------------------- Don't forget the magic of a 1/2 wavelength multiple of _any_ Z coax: It repeats the load Z at the far end. So, if you use a piece of 75 ohm that is 1 wave on 20, 1.5 on 15, and 2 on 10 (and .5 on 40 to boot), you dont have to do any matching - the Z at the TX end will be the same as the antenna Z. This magic length works out to ~60 ft with the hardline I've used. And, of course, 2x, 3x, etc of this length also works. For really long runs (many hundreds of feet), the bandwidth starts to decrease, but for up to 240' (1 wavelength on 80), It's worked fine even on 10M. Your best bet is to "tune" the hardline: Cut it many feet longer than you expect (say 70' or 130'...), then put a dummy load on the far end & look for the lowest frequency SWR minimum. This is the frequency where the line is 1/2 wave long. Then, cut bits off the cable till the SWR minima are wehre you want them. (The hardline I've used tends to have VF's of 85% to 88%). 73, Paul, NX1H -------------------------------------------------------------------------------- first, if you really want to match it you can't use the 'q' section because of the multiple bands. second, there is no use trying to match it since the swr caused by the 75 to 50 ohm transition is minimal, especially at hf. the only reason to try to tune that mismatch out is for some uhf activities with relatively long runs. i use 75ohm catv cable for runs up to 350' on 20m and about 300' on 10m with no problems and the measured loss is negligible. the only trouble i have had with 75ohm cable like that is on 160m where the antenna impedance was lower (probably 30-40 ohms), and the length of the cable was near an odd 1/4wave multiple. this transformed the low impedance up to something much higher (about 150 ohms) which when i brought it back to a 50 ohm line into the shack had higher than desired swr at them amp. a short piece of rg-11 in series transformed it back to a resonable value. 73, Dave KY1H Robbins@guid2.dnet.ge.com -------------------------------------------------------------------------------- Hi Charlie, just put a 75 ohm stub on each end of the hardline and don't tell the th-6 you are using 75 ohm cable. You and the antenna will never know the differnce. I am feeding 8 antennae with 75 ohm and I haven't tuned anything or used any kind to xformer. Gud luck. 73 Gary WB4FLB hext@wkuvx1.wku.edu -------------------------------------------------------------------------------- I don't think you need to worry about matching. I have monobnanders, 10-40 wire on 80 & 160 plus a KT34XA and a TA33....none of them care that it is 75 ohm CATV hardline. I don't think you'll have a problem. 73/Greg KO9Y -------------------------------------------------------------------------------- Just keep the feedline at a multiple of an full wavelength at 20 meters. Then 15 will be 3/2 wavelengths, and 10 will be 2 wavelengths. At 1/2 wavelength multiples the impedence seen at the transmitter end is the same as is at the antenna end. This of course involves factoring in velocity factor, so the physical length will be about 80% of the 300/F free space wavelength. You can include sections of RG-11 in making up the wavelength, to handle the flexible sections needed at the rotator and (if the length is appropriate) in the shack as well. If the length is not convenient at the shack end, make the hardline plus RG-11 section a multiple of a 20 meter full wave, and add 50 ohm flexible coax (any length) at the shack end to reach wherever you have to go. I used this successfully for several years until I came into some free 50 ohm hardline. 73 - Jim (K8MR) -------------------------------------------------------------------------------- (The following message appeared after the summary was posted to the CQ-Contest reflector, but I thought it should be included with this summary - KA9FOX) This is my first message to this reflector .... all of the others have been direct replies to specific individuals. However, I want to comment on some of the replies that were received, and summarized, by Charlie, KD5PJ/9. My comments have to do with SWR and Length of Feedline. Several comments were made regarding using line that is cut to a multiple of an electrical 1/2 wavelength so thet the load impedance will be mirrored at the transmitter end. One comment accompanying this idea was that if this is done then the impedance of the feedline becomes immaterial. Another comment was to the effect that, "You can always adjust the SWR at the transmitter by changing the overall length of the feedline". The 1/2-wavelength mirroring concept is certainly correct. However, neither it, nor anything else having to do with the length of the feedline will have the slightest effect on the SWR that exists on said feedline. Basic transmission line theory shows clearly that the ONLY thing that affects SWR on a line is the load at the end of the line. Everything else just affects the impedance the transmitter sees, but the SWR remains the same. Thus, a 75-ohm catv line, coupled to a pure 50-ohm resistive load, would yield a 1.5-to-1 SWR, regardless of feedline length. At electrical 1/2 wavelength intervals from the load, the impedance at that point on the line would be 50-ohms, but the SWR on the line would still be 1.5-to-1. As the additional comments so correctly pointed out, this slight mismatch combined with a fairly low loss line, would yield only a small additional signal loss. Changing feedline length to get a better "match" is fine, as long you don't get sucked into the idea that this is somehow affecting the SWR. 73, Wes ....N5WA wes@ns1.unicomp.net