Re: [Elecraft] KAT1 (K1) & end-fed halfwave wire antenna
Wow. Thanks to all for the wealth of information regarding this type of antenna. Plenty to consider! Deciding what and where to tradeoff for a given use is what makes each set up unique. I think I'd like a dipole/doublet for use when supports are available and an end-fed of some sort when I don't have that luxury. Luckily if there's room for 1 wire antenna there's probably room for two. Thanks again & 73! Mike ab3ap ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
RE: [Elecraft] KAT1 (K1) & end-fed halfwave wire antenna
FWI to the Group: Another form of the "end fed dipole" that I have been using was written up in August 1991 QST and has also been shown in some additions of the ARRL Handbook. It is a real dipole - 50 ohm input impedence. The first "side" is made as per standard formulas. The otherside is made up of coax whose center conductor is connected the directly to the end of the first side. This connection forms the middle of the antenna. A portion of the coax becomes the second side of the dipole, or more specifically, the outside of the coax shield becomes the second side of the dipole. The center conductor and inside of the shield form the feed line. Loops of coax placed the proper length away from the center electrically define the end of the second half of the dipole. The remaining coax can be of any length and becomes the feed line. There are two tricky parts to making these antennas. The first is making the joint at the middle mechanically strong enough so that it doesn't break when the antenna is strung. The second is getting the coax coil right so that its inductance and self capacitance resonate at the band in question. The Articles mentioned give good starting points, and an antenna analyzer makes tweaking the coil a snap, but it can be done just with an SWR meter. I have been camping with a 40m dipole. They are compact and easy to handle. It was great to be sitting under a tree at one end of the antenna operating and not out in the sun in the middle. Rob Brigham KC6ROC K2#5924-in progress "Ron D'Eau Claire" <[EMAIL PROTECTED]> wrote: >You are right about the improved efficiency of the end fed dipole (i.e. half >wave) antenna. From the standpoint of radiation, it's identical in all >respects to a center fed dipole. The only difference is that you're feeding >at a voltage loop instead of at a current loop. For portable use, that means >no feeder is needed and you can still get the high-current portion of the >antenna up "in the clear" - at least 1/4 wave up if you're able to run it >vertically or as an "inverted L" with the bend at the center of the wire. > >There are two problems with an end fed dipole. First, most automatic antenna >tuners (including the Elecraft tuners) cannot handle the very high impedance >of an end fed dipole. Secondly, such tuners do not isolate the rig from the >high RF voltage present at the end of the antenna. The whole rig tends to >"float" up to the high RF voltage at the end of the antenna which makes the >rig "hot" with RF. In modern rigs filled with logic chips, the RF can raise >havoc by producing false inputs to the logic here and there at random. In >extreme situations you can even feel the "bite" of the RF voltage when you >touch the rig, even at QRP power levels. Also, at a voltage loop, >capacitance to ground affects the tuning. Your body touching the rig easily >detunes the antenna system. > >The hot RF issue is the easiest to deal with, but makes the antenna more >complex. Ground the rig. It doesn't have to be a very efficient ground; just >something to reduce the RF voltage to an acceptable level. A 1/4 wave length >of wire isolated from ground and hooked to the case of the rig is perhaps >the simplest although all sorts of lengths of wire laid on the ground >usually work too. > >Matching to the high impedance at the end of a dipole is a little more >difficult to deal with using an automatic tuner. I currently use an end-fed >dipole in a temporary setup with my rig on the second floor of the house. I >use a manually-adjusted link coupled tuner that effectively decouples the >antenna from the rig. Unfortunately, such tuners are very difficult to >automate. It has two variable capacitors that must be adjusted and five coil >taps to adjust when moving from one band to another. Most automatic tuners, >including the Elecraft tuners, us an "L-network". It is an efficient and >flexible design, but because of the very high RF voltages that can be >present when the impedance is very high, the compact, automated versions >aren't designed to match the very high impedance at the end of a dipole. > >That's the bad news. The good news is that it's not necessary to use an >antenna exactly 1/2 wave long for good efficiency. While the 1/2 wave is >ideal, lengths longer or shorter, within reason, can still provide an >efficient antenna. As you move away from the half wave length, the impedance >drops and ground losses increase, so the idea is to find the length closest >to 1/2 wave that your ATU will still handle and which don't produce any >detuning effects when you touch the rig. > >Many folks do what you are considering: use a coil or a variable capacitor >in series with the antenna where it connects to the rig. A tapped coil is >more common because coils are less easily damaged in the field. The coil or >capacitor will change the impedance at the feed point so the ATU can find a >match. For highest efficiency, you want to start with an antenna a close
RE: [Elecraft] KAT1 (K1) & end-fed halfwave wire antenna
You are right about the improved efficiency of the end fed dipole (i.e. half wave) antenna. From the standpoint of radiation, it's identical in all respects to a center fed dipole. The only difference is that you're feeding at a voltage loop instead of at a current loop. For portable use, that means no feeder is needed and you can still get the high-current portion of the antenna up "in the clear" - at least 1/4 wave up if you're able to run it vertically or as an "inverted L" with the bend at the center of the wire. There are two problems with an end fed dipole. First, most automatic antenna tuners (including the Elecraft tuners) cannot handle the very high impedance of an end fed dipole. Secondly, such tuners do not isolate the rig from the high RF voltage present at the end of the antenna. The whole rig tends to "float" up to the high RF voltage at the end of the antenna which makes the rig "hot" with RF. In modern rigs filled with logic chips, the RF can raise havoc by producing false inputs to the logic here and there at random. In extreme situations you can even feel the "bite" of the RF voltage when you touch the rig, even at QRP power levels. Also, at a voltage loop, capacitance to ground affects the tuning. Your body touching the rig easily detunes the antenna system. The hot RF issue is the easiest to deal with, but makes the antenna more complex. Ground the rig. It doesn't have to be a very efficient ground; just something to reduce the RF voltage to an acceptable level. A 1/4 wave length of wire isolated from ground and hooked to the case of the rig is perhaps the simplest although all sorts of lengths of wire laid on the ground usually work too. Matching to the high impedance at the end of a dipole is a little more difficult to deal with using an automatic tuner. I currently use an end-fed dipole in a temporary setup with my rig on the second floor of the house. I use a manually-adjusted link coupled tuner that effectively decouples the antenna from the rig. Unfortunately, such tuners are very difficult to automate. It has two variable capacitors that must be adjusted and five coil taps to adjust when moving from one band to another. Most automatic tuners, including the Elecraft tuners, us an "L-network". It is an efficient and flexible design, but because of the very high RF voltages that can be present when the impedance is very high, the compact, automated versions aren't designed to match the very high impedance at the end of a dipole. That's the bad news. The good news is that it's not necessary to use an antenna exactly 1/2 wave long for good efficiency. While the 1/2 wave is ideal, lengths longer or shorter, within reason, can still provide an efficient antenna. As you move away from the half wave length, the impedance drops and ground losses increase, so the idea is to find the length closest to 1/2 wave that your ATU will still handle and which don't produce any detuning effects when you touch the rig. Many folks do what you are considering: use a coil or a variable capacitor in series with the antenna where it connects to the rig. A tapped coil is more common because coils are less easily damaged in the field. The coil or capacitor will change the impedance at the feed point so the ATU can find a match. For highest efficiency, you want to start with an antenna a close to 1/2 wavelength long as possible, and then add only the reactance (capacitance or inductance) needed to get a match; no more. The farther you go from 1/2 wavelength, the lower the antenna efficiency. That is, unless you can set up a good ground system. And that's why some portable operators choose to go to the trouble to rig a 1/4 wave wire, insulated from surrounding objects, as a "counterpoise" connected to the rig case. You need one for each band. You can use multiwire cable with each wire cut to the right length, but be sure to separate the last few feet of each wire from the others. The point about a 1/4 wave wire is that if you force one end to be "hot" (at a high impedance) by insulating it from everything around it, the other end you connect to the rig will show an impedance of about 35 ohms: low enough to be a decent RF ground. You must separate the ends from the other wires (and other objects) to keep them from detuning the wire. BTW, your friend is right about the "switch". That's part of an L-network like the Elecraft tuners use. In the Elecraft ATUs the switching done automatically. It's part of the relay chatter you hear going on while the ATU is searching for a match. Ron AC7AC -Original Message- The other day I posted an antenna question about making a K1 antenna that avoids halfwave multiples (& thanks again to the guys who replied offlist). From the calculations in that note and some modeling by another op it seems the ~18m/60' is probably the length to try there. But end-fed halfwave antennas have some advantages, especially the fact that as a voltage fed antenna they
Re: [Elecraft] KAT1 (K1) & end-fed halfwave wire antenna
Has anyone tried either method? (That is, direct connection of halfwave end-fed or tapping an inductor.) Hi Mike, This topic comes around every few months, so you can find a lot of knowledge - both theoretical and practical - by searching the TenTec and QRP-L archives for 'End Fed Halfwave' or 'EFHW'. Most EFHW antennas use either an impedance transformer wound on a toroid (Steve, AA5TB), a parallel LC with a tapped L (Don, W3FPR), or a series L / shunt C matching network (Steve, KD1JV). My apologies if I got the references wrong. All the above methods work. I have not done enough work with EFHW antennas myself to draw my own conclusion yet, but my *impression* is that it's a convenient antenna for portable use, but not as good as a conventional dipole (which is not as convenient to set up as a simple wire). I'm sure you'll get a variety of responses because this seems to be a favorite topic. 73, ed - k9ew ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
Re: [Elecraft] KAT1 (K1) & end-fed halfwave wire antenna
Hi Mike, Another option is to wind a transformer on a toroid with a 3:1 turns ratio which will give you a 9:1 impedance transformation. If your antenna has a high impedance, which an end fed half wave will naturally have, the transformer will drop the impedance down into the range of your auto tuner. The other advantages of this are, on other bands where your antenna impedance is lower your impedance will still be matchable for your tuner. Also, if you ground the low end of the primary and the secondary windings of the transformer, it should make your antenna quieter in terms of noise. I read somewhere that it makes your antenna less attractive to lightning by not having that positive static charge on it, but I am not sure about that part. I have an end fed half wave which I like, but I have a homebrew tuner. 73, Bob K6GGO Mike Markowski wrote: The other day I posted an antenna question about making a K1 antenna that avoids halfwave multiples (& thanks again to the guys who replied offlist). From the calculations in that note and some modeling by another op it seems the ~18m/60' is probably the length to try there. But end-fed halfwave antennas have some advantages, especially the fact that as a voltage fed antenna they have less of a need for a low impedance ground to keep rf losses low. On the other hand, they offer very high impedance to the tuner. The KAT1 manual says to look out for "rf problems" at exact half wavelengths but doesn't come out and say if it can or can't handle it (even if just at certain ranges of length). A longtime ham told me some ATUs used to have a switch to move a cap to one end of the matching network or another to offer a wider matching range. So if the KAT1 can handle the impedance, great. If not, maybe I can put a coil in series and tap it at the 50 Ohm point of the coil. I'm not sure if it's worth the effort, especially since it'd be nice to keep portable ops as simple as possible. But if there's a noticeable performace increase... Has anyone tried either method? (That is, direct connection of halfwave end-fed or tapping an inductor.) Thanks for any ideas. 73, Mike ab3ap ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
