may have it wrong
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Please correct if this is wrong, I am trying to use my KX1 but after reading the group emails, I may be doing this all wrong.
For frequency 7118, I took a length of bare copper wire 33 feet long, divided (that is cut it ) it in half, took one half to the center of the BNC connector on the side of the radio and the other half directly to the grounding rod attached to the house. but from reading the emails on the group list it now appears I need to have a insulated coax and I should not be connected directly to the radio with the two bare copper wires..... Mike _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Hi Mike,
> > For frequency 7118, I took a length of bare copper wire 33 feet long, > divided (that is cut it ) it in half, took one half to the center of > the BNC connector on the side of the radio and the other half directly > to the grounding rod attached to the house. but from reading the > emails on the group list it now appears I need to have a insulated > coax and I should not be connected directly to the radio with the two > bare copper wires..... You can certainly get away without any coax at QRP power levels, especially if you use an antenna that's close to a quarter wave long. It's OK to directly connect the wires to the KX1. But what you described won't work very well, because both wires need to be about 33 feet long at this frequency. By cutting one 33-foot piece of wire in half, you've made a 20-meter antenna :) The formula used to calculate the total wire length for the antenna is: 468 divided by the frequency in MHz So: 468 / 7.118 = 65.75 feet, or 65 feet 9" This is a *half-wavelength* of wire. To connect the antenna to your rig, you would then feed it in the center, where the impedance is close to 50 ohms (this matches the impedance of the rig's antenna jack). This of course involves cutting the wire in half. But you may want to add an extra inch or two at each end for use with antenna insulators, etc. The KX1 will be perfectly happy with this antenna. You can simply insert one wire into the center terminal of the rig and toss it into a tree (etc.), and connect the other wire to the rig's chassis and lay it on the ground. In fact this is what we designed the rig for: ad-hoc 1/4-wave or random-wire antennas fed without coax. Your SWR may as high as 3:1 if you don't trim the wire to length using an SWR bridge, but the KX1 can handle high SWR all day, so give it a go. If you want somewhat better performance, you can reconfigure the antenna as a dipole or inverted V, the higher off the ground the better. There are many examples in the ARRL Antenna Handbook, which I highly recommend. This, of course, will require coax, or balanced feedline and a balun (balanced-to-unbalanced converter; the KX1's antenna input is what we call "unbalanced," no reflection on the designer, yours truly ;) You might also consider using an antenna tuner. The optional KXAT1 will give you several advantages: - a good match for the transmitter, minimizing power loss - operation on multiple bands with just one wire antennna - power and SWR indication on the KX1's LED bargraph - the ability to peak up the antenna on receive manually outside of the ham bands (using the "G00-G31" selections in the KX1's ATU menu entry) Our new T1 ATU will also do the job (less the manual tuning capability), and can serve as an ATU for the KX1 and all other QRP rigs up to 20 watts. But you can't match the convenience of the KXAT1. 73, Wayne N6KR --- http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by mc-20
Mike asked:
Please correct if this is wrong, I am trying to use my KX1 but after reading the group emails, I may be doing this all wrong. For frequency 7118, I took a length of bare copper wire 33 feet long, divided (that is cut it ) it in half, took one half to the center of the BNC connector on the side of the radio and the other half directly to the grounding rod attached to the house. but from reading the emails on the group list it now appears I need to have a insulated coax and I should not be connected directly to the radio with the two bare copper wires..... -------------------------------------------- Your setup is FB Mike. There's absolutely no need to use a piece of coax unless you must have a transmission line to bring the antenna indoors through electrically noisy areas that might cause you trouble. A transmission line has only one job: move the RF between your rig the radiator (antenna) somewhere out in the clear (hopefully) with a minimum of pickup or radiation that might cause you problems. But every transmission line has a lot of considerations to avoid excessive loss. EVERY transmission line introduces losses - it's just a matter of the degree of loss. Well designed antennas can show lots of gain produced by having the radiator out in the clear and hopefully up high that far exceed the losses in the line, so the line is worth while. If you're outdoors or if you are sitting by a window in the house, especially if you're on an upper floor, by all means simply hook two wires to the KX1: one to the case and one to the center pin of the antenna connector. Run them out the window. Let the ATU tune up and you're all set. Ideally run the two wires in OPPOSITE directions as much as you can, or at least at an angle that has as a minimum a 90 degree angle between the wires. That's because if they are at too acute an angle they will try to act like an open wire transmission line and the radiation will be suppressed. Ideally on 40 you'd have two 33 foot lengths instead of two 16 foot lengths, but a 33 foot length will be close to a half wave on 20 meters and the ATU may not be able to match to it. So your choice of two 16 foot lengths is a FB compromise that should allow the ATU to find a match on all three bands. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by wayne burdick
Wayne N6KR wrote:
"...what you described won't work very well, because both wires need to be about 33 feet long at this frequency. By cutting one 33-foot piece of wire in half, you've made a 20-meter antenna :)" -------------------- A modeling program like EZNEC predicts that a 33-foot center fed wire will show a gain of about 6.9 dbi on 7 MHz and about 7.1 dBi on 14 MHz. That's essentially no difference at all. That's at a height of about 30 feet where the antenna's main lobe is straight up for short skip work on 40 and down around 30 degrees on 20 - useful for DX ing. The difference remains on the order of 0.1 or 0.2 db between 40 and 20 all they way down to under 15 feet, where ground absorption will start to show a significant loss on 40 due to the low height of the antenna. That miniscule difference in performance between a half-size and full size doublet agrees with what many handbooks, including the ARRL handbook, has claimed for many years. Of course, that assumes no difference in matching network losses, which I'd not expect a KX1 user to see using such an antenna without a feed line. (Of course, if a coaxial feeder is used, then some means to holding down the SWR on the feedline is very important to avoid losses there.) I was concerned that the tuner in the KX1 wouldn't be able to handle the high impedance of a 1/2 wave (33 ft) wire on 20 meters. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by mc-20
In a message dated 6/17/05 2:12:37 AM Eastern Daylight Time,
[hidden email] writes: > Wayne N6KR wrote: > > "...what you > described won't work very well, because both wires need to be about 33 > feet long at this frequency. By cutting one 33-foot piece of wire in > half, you've made a 20-meter antenna :)" > > -------------------- > > A modeling program like EZNEC predicts that a 33-foot center fed wire will > show a gain of about 6.9 dbi on 7 MHz and about 7.1 dBi on 14 MHz. That's > essentially no difference at all. > > Of course, that assumes no difference in matching network losses, which I'd > not expect a KX1 user to see using such an antenna without a feed line. (Of > course, if a coaxial feeder is used, then some means to holding down the SWR > on the feedline is very important to avoid losses there.) > > There's more to it, though: On 40 meters, the feedpoint impedance will look like a resistance of a few ohms and a reactance of several hundred ohms or more. Will the matching network be able to handle that, and do it without undue loss? 73 de Jim, N2EY _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Jim N2EY wrote:
On 40 meters, the feedpoint impedance will look like a resistance of a few ohms and a reactance of several hundred ohms or more. Will the matching network be able to handle that, and do it without undue loss? ---------------------------- The impedance at the center of a 33 foot doublet on 40 meters will be about 12-j850 ohms. I would expect the KX1's ATU to match that without problem. However, Wayne pointed out off the reflector that I was making a rash assumption: Mike did *not* say he had an ATU! Without the ATU it's very important to provide a decent match to the KX1 output of course. That's why Wayne said to use a self-resonant 66 foot dipole. So Mike, if you didn't include the KXAT1 internal ATU or a T1 external ATU, maybe this is the time to add one! It's not a requirement, but if you don't use one you'll need to make sure to have a low SWR at the rig with any antenna you use in order for it to work very well. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by mc-20
Mike,
I would like to point out 2 things - first as others have indicated, a 33 ft. wire will be resonant somewhere near 20 meters - for 40 meters, it should be about twice that long, and if you do not have a tuner, it should be pruned to resonance because the KX1 will not perform well unless the antenna jack has something close to a 50 ohm resistive antenna connected to it. I did get the impression that you connected one far end of your wire to a grounding rod - that is not what you should do - if you are trying to use a 1/4 wavelength counterpoise wire, the far end should be open, not grounded, and be aware that there will be a high RF voltage at the far end of this wire. Such a counterpoise wire becomes an integral part of the antenna system. If you feel you need anything grounded, ground the shell of the BNC connector on the KX1 with a short large conductor so as to create a low impedance ground path. 73, Don W3FPR > -----Original Message----- > > Please correct if this is wrong, I am trying to use my KX1 but > after reading the group emails, I may be doing this all wrong. > > For frequency 7118, I took a length of bare copper wire 33 feet > long, divided (that is cut it ) it in half, took one half to the > center of the BNC connector on the side of the radio and the > other half directly to the grounding rod attached to the house. > but from reading the emails on the group list it now appears I > need to have a insulated coax and I should not be connected > directly to the radio with the two bare copper wires..... > > Mike > No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.323 / Virus Database: 267.7.7/20 - Release Date: 6/16/2005 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by mc-20
In a message dated 6/17/05 11:03:26 AM Eastern Daylight Time,
[hidden email] writes: > Jim N2EY wrote: > > On 40 meters, the feedpoint impedance will look like a resistance of a few > ohms and a reactance of several hundred ohms or more. Will the matching > network > be able to handle that, and do it without undue loss? > > ---------------------------- > > The impedance at the center of a 33 foot doublet on 40 meters will be about > 12-j850 ohms. Yes but what was described wasn't a doublet up in the air. IIRC, he wasn't using a feedline, so one end of the wire is close to the ground and the rig. More like a semivertical with one radial. 73 de Jim, N2EY _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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