coax stub summary To: cq-contest@tgv.com A while ago I was looking for info on making/using coax stubs for filtering between two stations. Thanks to W1PH, VS6WO, KY1H, K3NA, K1VR, W0CP, K3LR, KR0Y, WA6OTU, WM2C, ON6TT, N4OGW, (and maybe a few others who got lost while cutting and pasting this together) for the hints and comments. Several cited the K2TR article in the May - June 1984 NCJ. Following is a collection of the various comments received (in no particular order) beginning with the original post. Mike N0BSH n0bsh@aol.com ----------------------------------------------------------- I'm looking for information on using coax stubs for filtering between the two stations. Some questions that come to mind include: 1) Lengths required for nulling various bands - open/shorted? 2) Can stuff like RG58 be used or should we stick with RG8 (will be running about 1KW on each station?) 3) Do they need to be made with 50 ohm coax or can 75 ohm be used (have some of both laying around?) 4) Can we put these on a coax switch and switch in the ones we need depending on band? 5) Is it worth all this or should we just get out the MC/Visa and buy the Dunestar filters? ----------------------------------------------------------- 1. Lengths required: This depends on the velocity factor of the coax being used to make the stub. In addition, you will have to have an appropriate serial coax impedance transformer between the point where the stub and antenna feed attach and the radio. Otherwise the radio will see a wierd impedance and potentially high SWR. The lengths of the serial matching coax sections will be different for each combination of antenna and band-stub. (If you are getting the impression that this is complicated, you are correct!) Either shorted or open-ended stubs can be used. However, shorted stubs are easier to build properly. Open stubs tend to leak at high powers (remember, that will be a very high voltage point at the end of the open stub!) 2. The quality of the stub as a filter depends on the loss of the coax. RG58 coax has high losses and you will lose many 10s of dBs of filtering. Even RG-213 is pretty marginal... (This is another problem for the use of stubs in an expedition...) 3. Effective stubs can be designed using any impedance of cable, although the lengths of the matching sections will change. 4. Coax switching of stubs is very complicated because the matching sections must also be switched. 5. It is simpler to buy the filters. You will get similar performance to reasonable-quality stubs (altho not as good as excellent stubs, and not at 1500 W power levels). Switching is a breeze. And Dunestar or ICE filters are smaller and lighter to carry with you to the site. ----------------------------------------------------------- The stubs are easy and cheap. I use RG62A/U that I bought by the pound from a metal salvage yard. Impedance of the stubs does not have to match the transmission line. RG62A/U is 93 ohm coaxial cable. Small cable is fine as it only has to carry the spurious signal. I use 6 dedicated feed lines for the 6 bands and have stubs permanently attached to each line. There is no reason not to use a switch to select the right stub. But if you forget to switch, the amplifier may be looking at a short circuit with the wrong stub. Stubs are not real high Q devices and seem to cover an entire band easily. ----------------------------------------------------------- An effective combo of stubs and filters allows almost any band/antenna combo to be on the air at the same time. ----------------------------------------------------------- Yes, stubs are worth it even if you use ICE or other tx filters I think. Most of the other filters are only exciter level so you put them between the exciter and amp, but they don't do anything for amp produced harmonics. In general the stubs are 1/4 wave shorted pieces of coax, except on 40m to null out 15m harmonics. If you are running kw's you want to use at least rg8x, preferably rg8 or rg-213. We melted down a stub using rg-59 here one contest. No, the impedance of the coax doesn't matter, but the lossiness of it does. Going from rg-58 to 3/4" hardline increases nulls from about 15 to about 40db. Using decent quality rg-8 gives good results. Yes, you can switch stubs, just remember that the size of the switch and any connecting coax pieces add to the length of the stub so shorten up the stub to compensate for it. ----------------------------------------------------------- As most expeditions involve dragging too much stuff through reluctant airline channels, I'd recommend buying the DuneStars. Yes, you may bandswitch stubs in and out. Yes, you may make them out of 75 ohm cable. But to make stubs that work, you MUST have a testing machine that will read out frequency (AEA, MFJ, network analyzer, etc.) RG-58 vs. RG-213: The less loss, the better the stub performance. Remember, DuneStars go between xcvr and amp. Stubs of RG-213 can go after amp. I use ICE bn xcvr and amp, stubs of .750 inch diameter between amp and antenna. ----------------------------------------------------------- Coax stubs are best at reducing broad band phase noise from nearby transmitters. They will provide about 20-25db attenuation on the next nearest band. The lower the loss of the coax used on the stub, the deeper and narrower the notch will be - take your pick from any coax that will handle the power. Note that stations have had stubs melt in the 'heat' of battle!. Better stick with 50 ohms... Many stations don't bother with stubs in multi-single - just good rx filters. ----------------------------------------------------------- There is a middle way, which is to use an ant tuner as a "tunable stub". This doesn't work in a m/m operation, but for what you guys are doing, it might be the best approach. I have used MFJ 16010 tuners for this, which cost about $30. I can get about 30 db of rejection on the offending band. Operationally, you simply switch to the offending band and tune the device for minimum band noise, and put little notation marks on it for reference during the contest. ----------------------------------------------------------- RG-142BU which is that fancy, teflon-on-the-outside, the-size- of-RG58-but-takes-5KW stuff that you find at swapmeets. Using small coax has it's drawbacks, however, becacause of it's higher loss. One thing to remember about coax stubs is: The greater the loss, the sloppier the skirts of the filter, and the shallower the null as you sweep it. In other words, hardline makes fabulous coax stub filters, and RG58 makes ones that only work so-so. RG213 or RG8 (same thing, really) have been found by practical experience to represent the best all around compromise. ----------------------------------------------------------- Noise bridge: Start with the coax a little longer than you expect it to be after you've measured it to a 1/4 wave including the velocity factor, then you use this noise bridge gizmo and sweep the coax with your receiver. Where the hiss goes away is where the stub is cut for. It was one of those technical marvel things where everyone else in the room dropped their jaws and gasped with amazement. ----------------------------------------------------------- We use 2 sets of ICE filters. They are cheap ($30) and work well. there are some buts: ...1 physical filter per band= a lot of filter switching (we built all of them in a box) ...everywhere I went, someone always blew up the 20m filter. get a spare ...200w only. ... we tested them in a lab, they DO filter, but only if the SWR is good. The higher the swr, the worse the filtering ----------------------------------------------------------- A comical note. At a multi/multi I know well, they did not follow my advise and insisted in making their own 2Kw band pass filters. They tested the filters with the boxes still open and they performed well. They filled the filters with foam and closed the boxes. First minutes of the contest: everyone blew the filters up: the foam did not dry! ----------------------------------------------------------- 1) All the stubs I have used are 1/4 wave in length...if you leave the end open, the other end will appear to be closed. If the end is closed, the other end will appear open. So for example, to put a 20m null on you 40 antenna, put on a 1/4 wave 20m stub, and leave the end open. Or you can put a 1/4 wave stub on your 20m antenna, and leave the end closed. One trick that I have used for open stubs is to put a small air-variable cap on the end, and make the stub just slightly shorter than 1/4 wave...then you can set the null exactly by twiddling the cap while listening to the receiver. 2) Don't use RG58...the better the coax, the deeper nulls you will get. 3) Doesn't matter...I'm currently using 75 ohm double-shielded RG- 11 that was made for computer networks. 4) Yes, that is what I do...put a T connector on top of the switch. Be sure to account for the switch changing the length of the stub (use something like a noise bridge to cut them to length). 5) My current setup...I have 4 1/4 wave stubs on a coax switch to use with an all-band dipole. The stubs are closed on the end, so I switch in the one that is 1/4 wave on the band I'm operating on. There is some improvement, but not that impressive (my antennas are very close together). We played around with some of the ICE band filters at FD, and they seemed much more effective. Probably would also be lighter to take filters than lots of coax stubs on a dxpedition! ----------------------------------------------------------- If you wanna really do it right, you need bandpass filters and stubs! ----------------------------------------------------------- 1) Length is critical. Use a network analyser or other device to tune the stubs. Stubs are very frequency sensitive... you'll need one for SSB, and another for CW if you are trying to squeek out evry last dB. 2) Power was not the problem. RG58/RG8X did not (in my case) provide nearly the same attenuation per null as the RG8/213 did. 3) Suspect you can use 75 ohm. You will already have a small impeadence bump when you tap for the stub. 4) Yep. Be careful, for the coax switch adds enough length to the effective length of the stub to change the null outside your desired band segment! 5) Good question. All depends if you want to do the filtering before or after the amps. Amps are known for suprious emmisions. The stubs will help cure that, Dunstar/ICE won't. If you buy new RG213 for 160 thru 10, you come out about even with a set of ICE's. ----------------------------------------------------------- Thanks again to all for the good info. CU from PJ8Z in CQ WW SSB 94! Mike N0BSH n0bsh@aol.com