Practical use of antenna, tuner, etc by new tech
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| Oct 18th 2012, 22:00 | |
WA1UKGTotal Topics: 0 Total Posts: 0 |
I'm a soon to be tech (again) - Last active over 40 years ago. I intend to do CW (QRP) to get my speed up, then move up to general or ?? (depending on available time). I will probably build a small CW xmitter to use with a separate receiver, or buy something like the You Kit HB-1B. I have been reading many articles, but I'm still not sure if my antenna installation will be sufficient. My question has to do with the practical use of an antenna tuner. I can pretty easily hang a small whip onto a chimney, or maybe string 35' feet (total) of dipole between two chimneys. I expect I will need about 35 feet of coax to get inside the house to attach to the tuner. The TX will be close to the tuner. Will I need to prune a dipole? What coax should I use for a dipole? What coax should I use for a whip? Will a simple tuner take care of most frequencies? Any recommendations for tuners? I greatly appreciate any input. |
| Oct 18th 2012, 23:42 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
I suggest starting out with a 40 meter dipole, unless you like to operate at night in the winter and have big trees for an 80 meter dipole. Chances are, it will operate adequately without pruning, though you may get a slightly better signal with pruning. When running very low power, I've found it better to have a good antenna on one band that compromise antennas on several bands. RG-58 or RG-8X will work just fine for a single band dipole. You can get a significant improvement for by going to larger LM4-400.9913 if you intend to use a tuner to operate the dipole on other bands. For a multiband dipole, I'd suggest cutting the antenna for 30 meters. But, a tuner in the shack with a coax feedline compromises signal strength whenever the SWR on the line is high--it is quite common to lose S units worth of signal on transmit. This is why remote autotuners have become popular despite the expense and hassle of having something outside, exposed to the weather. Our product review page has reviews of tuners. Alternately, you can find reviews on eham.net. But,if you are just starting out, you are better off with a single band tuned dipole antenna and no tuner. Zack Lau W1VT ARRL Senior Lab Engineer |
| Oct 19th 2012, 00:23 | |
WA1UKGTotal Topics: 0 Total Posts: 0 |
Thank you Zack. Is the HB-1B something I could use to tune for contacts?
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| Oct 19th 2012, 01:00 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
I've made contacts with a number of different QRP rigs, but have not used the HB-1B--I had to look it up to see what bands it operated on. It does take more skill to make contacts with low power--with high power it is much easier for the other operator to figure out what you are doing--when signals are very weak it becomes much harder to deals with mistakes and faux pas. Zack Lau W1VT ARRL Senior Lab Engineer |
| Oct 19th 2012, 01:42 | |
WA1UKGTotal Topics: 0 Total Posts: 0 |
A 30 meter dipole will be roughly 55 ft of wire. I'm not sure I can get that in a straight line, so maybe an inverted U arrangement? I can mount some pvc on each chimney and run the end wires inside. That would put the horizontal wires at about 25 to 30 ft above ground. If I want good flexibility to different bands it sounds like I will need an autotuner. Should it be mounted right at the junction of the dipole? Is there any particular type that I should research? The peak of my roof is about 18 ft off the ground, and is 70 feet long. Maybe I can install some standoffs and run a wire along the peak? If I did that it would pass about 12" from the cooler. |
| Oct 19th 2012, 14:15 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
As a rule, most HF antennas are compromises. As a beginner, I'd suggest putting up what you can, trying it out, and seeing if the performance meets your needs. Electrically, it is best to mount the autotuner at the feedpoint of the dipole, but it may make more sense mechanically to run open wire to the feedpoint. Generally, it is best to get antennas as far away from the roof as possible, but folks have had acceptable results with antennas close to the roof. A big factor is band conditions--it is quite like the weather in that it constantly changes--someone willing to wait for good conditions is similar to someone willing to wait for good weather--your chance of having an enjoyable vacation is much greater--your chance of making contacts with other stations is much greater on the days with good band conditions. Zack Lau W1VT ARRL Senior Lab Engineer |
| Oct 23rd 2012, 01:57 | |
WA1UKGTotal Topics: 0 Total Posts: 0 |
By using some fascia boards from adjoining areas I can get at least 70 feet strung up on the side of my house. If a 40m dipole has one or more 90 deg bends at the ends of a "mostly" straight 50 foot section of the antenna, how will that affect the operation? Will I have to add or subtract wire? (shaped sort of like this> ____/``````\_ ) |
| Oct 23rd 2012, 11:27 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
Bending the ends like that typically means you need to add wire to properly resonate the dipole. But, one shouldn't put too much stock into theoretical analysis. Antennas are heavily influenced by proximity to ground and metal objects closer than 1/2 wavelength--in practice--one can often get an antenna resonant faster by merely cutting and measuring until it works. You can also analyze non-standard antenna installations by modeling them with computer software. This is particularly useful if you have a lot of time to investigate different options--or happen to have a gift for solving problems. Zack Lau W1VT ARRL Senior Lab Engineer |
| Oct 25th 2012, 23:29 | |
KB0HAETotal Topics: 0 Total Posts: 0 |
Hi. Just a few thoughts here. A 40m dipole is just under 66 feet if cut for 7.1 Mhz. You might try the inverted "U" with the legs going to the corners of the roof instead of vertically down the PVC pipes. Remember that the PVC will sway somewhat in the wind. If you stretch the wire too tight, it can snap if you do not allow for movement of the supports in the wind. The antenna does not have to be in a strait line. The ends can be bent if necessary. Remember that the higher your antenna is, the better, and the farther from objects (especially metal objects) the better. You might have to make some compromises on the height or distances from objects. Sometimes the best way is to get the antenna in the air and go from there. |
| Oct 31st 2012, 20:32 | |
WA1UKGTotal Topics: 0 Total Posts: 0 |
After a lot of head scratching, talking to myself, and just generally loosing contact with my surroundings, I decided that the best installation is probably the simplest. In my front yard, next to the road, is a 25 ft palm tree. 69 ft from the top of the palm tree (straight shot) is the peak of my house. I will hang a 40m dipole without a balun between the two. I will be using CW on 40 and 15m. QUESTION: If the ends of the wire are insulated, should my resonant length calculations include the twisted wire attached to the end insulators? To insure the wire not coming loose I expect to twist at least 6 or 8 inches (each end) thru the insulator back around the primary wire. If the wire is bare is it then just the distance to the end? |
| Nov 1st 2012, 17:15 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
The dipole length is the end to end distance. I've resonated my 20M dipole on 10 and 15 meters by bending back the insulated wire so that the lengths are for 10 and 15 meters. Zack Lau W1VT ARRL Senior Lab Engineer |
| Nov 6th 2012, 00:52 | |
WA1UKGTotal Topics: 0 Total Posts: 0 |
Is it acceptable practice to let some extra wire hang down from each end of the antenna to make pruning easier (or maybe add some wire without unwrapping the existing)? I recall reading somewhere about someone that added quick connects to make changing frequencies easier by using different lengths of dangling wire.. |
| Nov 6th 2012, 15:00 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
It is a bad idea to have dangling wire that can be easily reached--someone could get a RF nasty burn by touching the wire. Zack Lau W1VT ARRL Senior Lab Engineer |
| Nov 7th 2012, 20:22 | |
aa6eTotal Topics: 0 Total Posts: 0 |
Zack is right when it comes to safety. However, sometimes a few inches or feet make all the difference. The general rule is that you should cut wire antennas a little long and then trim them as necessary to get the resonance where you want it. It's easier to trim down length than to add length. But if you need a modest extra length, a dangling wire may be the easiest thing, though not the prettiest. You don't want it blowing around in the wind too much either, or your resonance will move around, changing your SWR, etc. Another approach, if limited space is the problem, is to make a bent wire, with the end pointing off in some other direction, supported by another rope. A straight wire may be slightly better in terms of radiation pattern and efficiency, but bent dipoles (or V's or other geometries) can work fine, especially if they are bent symmetrically so the feed currents are balanced. 73 Martin AA6E |
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