Antenna recommendation needed
On 18/01/2015 20:55, Bruno wrote:
Hi all, I've had a planning application for a tower turned down which is a real pain. My options are now very limited. I'm in a bungalow with a roof apex height of 18 feet and I'm not allowed to put up anything much above this roof line it transpires, so inverted vees that need to be mounted high up at the feed point like G5RVs are not possible for me. My garden boundaries limit the length of any wire antenna I might wish to put up to about 120 feet overall. On the plus side, though, the QTH is several hundred feet ASL close to the top of the hill and am not overshadowed by any trees or buildings to speak of. What's my best choice for an HF antenna under these circs? (I mostly do CW on 20m & 40m, but would like a bit more band choice ideally if poss). Many thanks. Many thanks. Your QTH is virtually identical to mine - max height of 18 foot, longest clear run about 85 foot. I use a doublet - just two 57 foot wires forming the radiator/feeder. The ATU is 16 turns on a two inch former, tuned by a 250pF variable and link coupled (about 7 turns) to the rig via a 1000pF variable. This tunes/matches 80, 60, 40 and 20m. The two capacitors are actually double 500pF caps - one with sections in series, the other with sections in parallel. If local interference is an issue it is important to keep the radiating section as straight and as horizontal as possible. PA |
Antenna recommendation needed
On Monday, January 19, 2015 at 12:31:04 AM UTC-6,
For 40m NVIS within 500 miles or so, the low dipole will likely smoke most verticals. Of course it will as all the energy will go straight up while the vertical main lobe is at about 30 degrees. The important factor is the height in wavelengths, not feet. As the height drops below a half wavelength the main lobe goes vertical on a dipole (or any horizontal antenna) rapidly. Sure, but there is still enough at the low angles to make some DX contacts. And he'll have a decent signal to the stations not so far away on 40m. I guess he'll have to decide what he wants to lean to. Whatever he decides is unlikely to excel at both, and will be a compromise. If local communications is your goal, then an NVIS antenns is what you want. However as the frequency increases above about 8 MHz the probablity for success decreases and drops to near zero at 30 MHz. For 20 meters it is a crap shoot with less than good sunspot activity. The performance at low angles won't be quite as grim as you might expect. IE: I had no trouble working 15m DX with a -10 ft high dipole. 20 ft up is nearly a 1/2 on 15m. And a 20m dipole at 20 ft will be quite decent for average use being over a 1/4 wave up. Will be good stateside, and usable for DX. I like converted CB ground planes for 10m. Good space wave for local, and good for DX. Not much close in sky wave stuff to work on that band. I don't really see how a dipole requiring three supports, two at best, can be considered simpler than a vertical with one support. A vertical generally needs radials unless it's a "1/2 wave" type design. And even good verticals can be quite lackluster for close in work compared to a low dipole on 40m. Good at night to DX though. But like I say, he's gotta decide what he wants to concentrate on, and go from there. If he has trees to tie dipole legs to, he really only needs one support for the apex. And one can also string them between trees to where no man made supports are needed at all. But I don't know what trees he has available. I usually have one mast for the apex, and tie off to trees or whatever. At the dirt patch, I use a oak tree as the apex support, and tie off to other trees. I shoot a weighted line up into the apex tree, and run it over a tall branch. Then I pull the dipole and coax back up into the tree with the wire I shot over the tall branch. When I go home, I let the wire loose, and back down it all falls. I used to leave it there all the time, but the critters were eating my coax into shreds, so I had to quit that. :/ And the tuner/ladder line fed dipoles is another option if one wants all bands with one antenna. |
Antenna recommendation needed
On 18/01/15 20:55, Bruno wrote:
I've had a planning application for a tower turned down which is a real pain. My options are now very limited. I'm in a bungalow with a roof apex height of 18 feet and I'm not allowed to put up anything much above this roof line it transpires, so inverted vees that need to be mounted high up at the feed point like G5RVs are not possible for me. My garden boundaries limit the length of any wire antenna I might wish to put up to about 120 feet overall. On the plus side, though, the QTH is several hundred feet ASL close to the top of the hill and am not overshadowed by any trees or buildings to speak of. What's my best choice for an HF antenna under these circs? You could have two...a horizontal, as suggested by others, and then there's this.... Consider is using the slope of the hill to your advantage. How close to it are you? Over what arc of the compass does the slope cover? My thinking is that you should consider ground-mounting a vertical as close to the slope as possible. The down-slope will pull down the vertical radiation lobe to perhaps all but the horizontal, giving you effective gain over a vertical operating over level ground together with the prospect of long hop-length DX from the low angle. The difference in gain can be useful to significant., and the reduced number of hops can reduce path losses. Get a great-circle map and see what sector of the world is covered by the arc of the slope; you'll get the best reports from within this arc. The vertical need not be high at all. For the lower bands you could try the following set-up; you could make it in an hour or so. An auto-tuner would make this a breeze, but you can put together the following from your junk box. Wind a coil having an inductance of 0.25 x lambda for the band of interest. For 40m this would be 0.25 x 40 = 10 microH. I make my coils from flex stripped from scrap mains cable. You'll also need a variable capacitor of about 1pF per lambda, but this isn't critical. Connect this across the coil. Wind on a 4-turn link at the 'bottom' end of the coil connected to 50 ohm coax and connect to your rig. To the 'top' end of the coil, connect your vertical. Mine is 18' of wire taped to a roach pole, for the lower bands. To the bottom end connect four radials, each as long as the vertical is high. Connect to a screwdriver or very short rod earth via an RF choke or 1k resistor. Do not bury the radials or connect a massive earth, this one is to bleed static, nothing more. The radials will couple the RF to ground. It's my experience that when the length of the vertical becomes a significant fraction of the wavelength (say 1/10th), the system begins not to work so well - the solution is to tap the aerial connection down the coil, but this adds complexity. Keep the vertical section short and enjoy the simplicity. This is a try-out of the possibilities of your QTH, after all. I find that this set-up is very non-critical, and with a 200pF variable and the right inductance it will tune over two bands, say 160/80, or 80/60/40. Above these bands the link's turns will need to be reduced and the system self-capacitance can be a nuisance, so perhaps an auto-tuner would be better. Your output power will be limited by the capacitor's capability voltage-wise. The only drawback is retuning for each band, as the tuner is located at the base of the antenna. I use mine in the portable mode, so operating near the base of the antenna isn't a problem. Using a similar set-up on a short (200') slope facing East, I can work all of Russia, the Mediterranean, and Asia from the UK with good reports and running less than 10W, on 17 and above (for which I use an auto-tuner). I don't work 20m, never liked the band much, and below that, the slope isn't really long enough to pull the vertical lobe down. Hopefully, yours will be. -- Spike "Hard cases, it has frequently been observed, are apt to introduce bad law". Judge Rolfe |
Antenna recommendation needed
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Antenna recommendation needed
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Antenna recommendation needed
John S wrote:
On 1/20/2015 12:30 PM, wrote: wrote: snip A vertical generally needs radials unless it's a "1/2 wave" type design. And even good verticals can be quite lackluster for close in work compared to a low dipole on 40m. Good at night to DX though. But like I say, he's gotta decide what he wants to concentrate on, and go from there. You are all over the place here and mixing apples and oranges. In what way? Seems perfectly reasonable and informative to me. A vertical has a low elevation angle. You will have to describe the conditions for that to be a truthful general statement. Do you mean ground mounted with buried radials, radials lying on the surface, elevated verticals with radials, or half-wave verticals with no radials? Most people mean ground mounted when they use the term vertical and ground plane for an elevated vertical. I meant ground mounted and the radials or lack of them is essentially irrelevant. Any antenna that has a low elevation angle is "better" for distant communications than an antenna with a very high elevation angles. I think that is agreeable by all. I would certainly hope so. A horizontal antenna less than a half wavelength in height has a vey high elevation angle. At .3 lambda it is 48 degrees, at .25 lambda it is 62 degrees, and at .2 lambda it is 90 degrees, i.e. straight up. According to the simulation programs, that is probably true. As well as basic physics. But, that is the angle of *maximum* radiation. How much signal is available at the 3dB, 6dB, 10dB, 20dB angles? If you are running 100 watts, you still have 1 watt available at the 20dB angle. I would be glad to run the numbers for you. Do you want it as a comparsion between a vertical with no radials, a vertical with radials, both over average ground, and a vertical with perfect ground? This is called an NVIS antenna which is "better" for local communications out to about 500 miles or so on the lower bands. NVIS communcations is nearly nil much above 8 MHz at other than sunspot peaks. We are currently past the peak of the current sunspot cycle and heading for a minimum. Can you supply support to some other source for this conclusion? I assume you mean for NVIS communications and not the sunspot cycle: http://en.wikipedia.org/wiki/Near_ve...idence_skywave http://www.w0ipl.net/ECom/NVIS/nvis.htm http://www.w5jck.com/nvis/W5JCK-NVIS...esentation.pdf https://www.txarmymars.org/.../NVIS-...and-Design.pdf http://www.qsl.net/wb5ude/nvis/ http://home.centurytel.net/w9wis/NVIS.html Want more? A vertical antenna does not "need" radials, but it's performance is improved by radials. It depends on the design. The design is a metal tube about .25 lambda long mounted close to the ground and fed at the bottom end. A vertical antenna over average ground has a main lobe at about 30 degees and a gain of about 1 dBi. Please supply configuration of the antenna as mentioned above. A metal tube about .25 lambda long mounted close to the ground and fed at the bottom end. A vertical antenna over perfect ground has a main lobe at 0 degrees and a gain of about 5 dBi. Again, please supply configuration of the antenna as mentioned above. A metal tube about .25 lambda long mounted close to the ground and fed at the bottom end. In a typical urban setting where there is landscaping and irrigation, the ground is most likely average to very good unless you are in the middle of a desert, which means even without radials the vertical's gain is going to be a few dBi. Adding a few radials will improve the gain and lower the main lobe but are not absolutely necessary. I think this is unsubstantiated unless you can supply supporting documentation. Trivially demonstrated by any antenna simulation program. Or are you looking for numbers on ground conductivity? Google ground conductivity Want to measure it yourself, read this: http://www.technik.dhbw-ravensburg.d...ductivity.html Most verticals will get put in the middle of a lawn and it is trivial to take a lawn edger or weed wacker and cut a groove in the grass down to the dirt and staple down some radials. In such a setting you do NOT need the 120 radials of a commercial broadcast station with it's antenna in a barren field. Most verticals will be installed depending on the installers resources, abilities, and present knowledge of antennas. The same can be said for making cookies; so what? One of the purposes of this group is the dissemination of antenna knowledge. I make no assumptions about the readers other than the ability to read. If he has trees to tie dipole legs to, he really only needs one support for the apex. The bottom line is a horizontal antenna needs three supports. Actually, a 40 meter dipole can use two supports (trees) and can work the US easily with a 50W transmitter. Sure, if you use very heavy wire for the dipole and very light feed line. BTW, most amateur rigs these days put out 100 Watts. -- Jim Pennino |
Antenna recommendation needed
On Tuesday, January 20, 2015 at 12:31:04 PM UTC-6, wrote:
wrote: snip A vertical generally needs radials unless it's a "1/2 wave" type design. And even good verticals can be quite lackluster for close in work compared to a low dipole on 40m. Good at night to DX though. But like I say, he's gotta decide what he wants to concentrate on, and go from there. You are all over the place here and mixing apples and oranges. Yes, because you are dealing with apples and oranges in the real world, and until he comes back and describes what mode of operation he prefers most of the time, I include both options. A vertical has a low elevation angle. And not much at the higher angles. Which will make it fairly poor compared to a low dipole when used for general 40m NVIS use, say in the daytime. Any antenna that has a low elevation angle is "better" for distant communications than an antenna with a very high elevation angles. Sure, but until he comes back and describes his usual mode of operation, I'm not sure if that is what he wants at all times. A horizontal antenna less than a half wavelength in height has a vey high elevation angle. At .3 lambda it is 48 degrees, at .25 lambda it is 62 degrees, and at .2 lambda it is 90 degrees, i.e. straight up. I've modeled and used antennas for many years.. I know the usual patterns of the various options. This is called an NVIS antenna which is "better" for local communications out to about 500 miles or so on the lower bands. NVIS communcations is nearly nil much above 8 MHz at other than sunspot peaks. We are currently past the peak of the current sunspot cycle and heading for a minimum. The angles used for 20m stateside can be fairly high. I know from experience that a 20m dipole at 20 ft will work quite well for average distances. It will be quite good within 1500 miles, and still quite usable at DX distances. It will generally be a good performing antenna for overall use. And if the band condx are heading down as you say, he would more likely be on 40 and 20, than the higher bands. Will he work 40m in the daytime? If so, he will want to be on the low dipole, rather than the vertical. Would be like comparing fresh cut oranges to rotten apples. :/ A vertical antenna does not "need" radials, but it's performance is improved by radials. Sure. But if he's over med to poor ground, the losses can be substantial. I'm over quite good ground being on the Gulf Coast, and I had a full size 32 ft tall 40m vertical ground mounted with 32 radials. It was OK, but no real DX buster. At all.. I then raised the antenna to 36 ft at the base, and used 4 sloping radials as a ground plane. That antenna absolutely smoked the ground mount antenna to DX. So the ground radials obviously were a factor, and I'm over pretty good ground. A vertical antenna over average ground has a main lobe at about 30 degees and a gain of about 1 dBi. And? A vertical antenna over perfect ground has a main lobe at 0 degrees and a gain of about 5 dBi. I don't know anyone who lives on perfect ground. :| In a typical urban setting where there is landscaping and irrigation, the ground is most likely average to very good unless you are in the middle of a desert, which means even without radials the vertical's gain is going to be a few dBi. Adding a few radials will improve the gain and lower the main lobe but are not absolutely necessary. That is true for the area I live on, and obviously the number of radials made quite a difference in my case. Raising the antenna let me use less radials to equal a certain number on the ground. I was using 32 on the ground for only so-so DX performance. The four I used at 36 ft were equal to about 60 or so on the ground at that height in wavelength on 40m. Most verticals will get put in the middle of a lawn and it is trivial to take a lawn edger or weed wacker and cut a groove in the grass down to the dirt and staple down some radials. In such a setting you do NOT need the 120 radials of a commercial broadcast station with it's antenna in a barren field. I'd want a decent number to really brown the food. And you have to consider the price of wire these days. Copper wire can add up to $$$$. If he has trees to tie dipole legs to, he really only needs one support for the apex. The bottom line is a horizontal antenna needs three supports. Not if you use two supports for the whole antenna, and let the feed line drop from the center down to the ground and then to the shack. He doesn't actually have to let it drop to the ground, but in the case of coax fed, I prefer that in case of a lightning strike. And snubbing the shield to a ground stake at that point, even better. I like metal masts as an apex support, as it will usually take the strike and route it to ground, instead of the antenna, as long as the mast extends slightly over the feed point of the antenna. snip And the tuner/ladder line fed dipoles is another option if one wants all bands with one antenna. As is a vertical piece of aluminum tubing of whatever height you can put up with an ATU at the base of it. Sure, I never said it wasn't. Almost a gazillion options in the wide wide world of antennas. :) But until he comes back and describes his usual mode of operation, I feel it's best to include all options, and not just limit it to verticals. |
Antenna recommendation needed
On Tue, 20 Jan 2015 16:09:25 +0000, Spike wrote:
How close to it are you? Over what arc of the compass does the slope cover? Good point. I'd forgot to specify that. I'm about fairly close to the summit and the ground drops away mainly to the SSE with the coast about a mile away. From here I can see couple of miles out to sea. Since I'm in the centre of the British Isles, I'm in a great position for getting good signals to and from South Africa - but not really anywhere else! :-( |
Antenna recommendation needed
wrote:
On Tuesday, January 20, 2015 at 12:31:04 PM UTC-6, wrote: wrote: snip A vertical generally needs radials unless it's a "1/2 wave" type design. And even good verticals can be quite lackluster for close in work compared to a low dipole on 40m. Good at night to DX though. But like I say, he's gotta decide what he wants to concentrate on, and go from there. You are all over the place here and mixing apples and oranges. Yes, because you are dealing with apples and oranges in the real world, and until he comes back and describes what mode of operation he prefers most of the time, I include both options. What he prefers is irrelevant to a general discussion about how antennas work. A vertical has a low elevation angle. And not much at the higher angles. Which will make it fairly poor compared to a low dipole when used for general 40m NVIS use, say in the daytime. Obviously. Any antenna that has a low elevation angle is "better" for distant communications than an antenna with a very high elevation angles. Sure, but until he comes back and describes his usual mode of operation, I'm not sure if that is what he wants at all times. What he prefers is irrelevant to a general discussion about how antennas work. A horizontal antenna less than a half wavelength in height has a vey high elevation angle. At .3 lambda it is 48 degrees, at .25 lambda it is 62 degrees, and at .2 lambda it is 90 degrees, i.e. straight up. I've modeled and used antennas for many years.. I know the usual patterns of the various options. This is called an NVIS antenna which is "better" for local communications out to about 500 miles or so on the lower bands. NVIS communcations is nearly nil much above 8 MHz at other than sunspot peaks. We are currently past the peak of the current sunspot cycle and heading for a minimum. The angles used for 20m stateside can be fairly high. I know from experience that a 20m dipole at 20 ft will work quite well for How high in wavelengths is 20 feet at 20M and how does that relate to what I have already said about antenna height? average distances. It will be quite good within 1500 miles, and still quite usable at DX distances. It will generally be a good performing antenna for overall use. And if the band condx are heading down as you say, he would more likely be on 40 and 20, than the higher bands. I said we are heading for a sunspot minimum, which means that NVIS communications is gettting more and more unlikely above 8 MHz. Will he work 40m in the daytime? If so, he will want to be on the low dipole, rather than the vertical. Would be like comparing fresh cut oranges to rotten apples. :/ What he prefers is irrelevant to a general discussion about how antennas work. A vertical antenna does not "need" radials, but it's performance is improved by radials. Sure. But if he's over med to poor ground, the losses can be substantial. I'm over quite good ground being on the Gulf Coast, and I had a full size 32 ft tall 40m vertical ground mounted with 32 radials. It was OK, but no real DX buster. At all.. If, if, if. The reality is most people live on average to good ground if for no other reason than they landscape it. I then raised the antenna to 36 ft at the base, and used 4 sloping radials as a ground plane. That antenna absolutely smoked the ground mount antenna to DX. So the ground radials obviously were a factor, and I'm over pretty good ground. The biggest factor is that you elevated it. Here are the numbers for a ground plane with 45 degree radials for various heights: Height is in wavelengths above ground for the bottom of the radials and @ elev is the elevation angle of the main lobe. Height gain @ elev 0.01 0.78 21 0.10 1.32 18 0.15 1.50 17 0.20 1.59 15 0.25 1.63 15 0.30 1.63 14 0.35 1.62 13 0.40 1.63 13 0.45 2.02 43 0.50 2.37 40 0.55 2.64 38 0.60 2.86 36 0.65 3.03 34 0.70 3.15 32 0.75 3.21 30 0.80 3.22 29 0.85 3.18 27 0.90 3.16 9 0.95 3.29 9 1.00 3.42 9 1.05 3.54 9 1.10 3.67 9 1.15 3.81 8 1.20 3.95 8 1.25 4.07 8 A vertical antenna over average ground has a main lobe at about 30 degees and a gain of about 1 dBi. And? And it is a fact. A vertical antenna over perfect ground has a main lobe at 0 degrees and a gain of about 5 dBi. I don't know anyone who lives on perfect ground. :| Neither do I but it is the reference for being as good as one can get, i.e. the best you could possible do with a huge number of radials. In a typical urban setting where there is landscaping and irrigation, the ground is most likely average to very good unless you are in the middle of a desert, which means even without radials the vertical's gain is going to be a few dBi. Adding a few radials will improve the gain and lower the main lobe but are not absolutely necessary. That is true for the area I live on, and obviously the number of radials made quite a difference in my case. Raising the antenna let me use less radials to equal a certain number on the ground. I was using 32 on the ground for only so-so DX performance. The four I used at 36 ft were equal to about 60 or so on the ground at that height in wavelength on 40m. Which is, for a lot of people, a well known phenomenon. You get more gain by elevating the antenna. You also have the effect that 4 elevated radials, even if only elevated a very small distance, are equivelant to a great number of buried radials, i.e. more closely approximates the perfect ground that doesn't exist. Most verticals will get put in the middle of a lawn and it is trivial to take a lawn edger or weed wacker and cut a groove in the grass down to the dirt and staple down some radials. In such a setting you do NOT need the 120 radials of a commercial broadcast station with it's antenna in a barren field. I'd want a decent number to really brown the food. And you have to consider the price of wire these days. Copper wire can add up to $$$$. 14 AWF THHN wire brand new from Lowes costs about $0.09/foot and will work just fine for radials. If you are really cheap, you can find surplus wire at swap meets for much less than that. If he has trees to tie dipole legs to, he really only needs one support for the apex. The bottom line is a horizontal antenna needs three supports. Not if you use two supports for the whole antenna, and let the feed line drop from the center down to the ground and then to the shack. He doesn't actually have to let it drop to the ground, but in the case of coax fed, I prefer that in case of a lightning strike. And snubbing the shield to a ground stake at that point, even better. I like metal masts as an apex support, as it will usually take the strike and route it to ground, instead of the antenna, as long as the mast extends slightly over the feed point of the antenna. Sure, like I said and which you snipped, if you use heavy wire for the dipole and a light feed line. You are still at at least two supports. snip And the tuner/ladder line fed dipoles is another option if one wants all bands with one antenna. As is a vertical piece of aluminum tubing of whatever height you can put up with an ATU at the base of it. Sure, I never said it wasn't. Almost a gazillion options in the wide wide world of antennas. :) But until he comes back and describes his usual mode of operation, I feel it's best to include all options, and not just limit it to verticals. What he prefers is irrelevant to a general discussion about how antennas work. And I did NOT limit the discussion to verticals, I compared low mounted horizontal antennas to vertical antennas. -- Jim Pennino |
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