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#11
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"David" wrote in message ... So for the 1/4 wave (81mm) section of the antenna the loss would be about 0.05dB ? at 915 MHz. 20dB/100' MATCHED line loss. W4OP |
#12
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David wrote:
So for the 1/4 wave (81mm) section of the antenna the loss would be about 0.05dB ? at 915 MHz. Depends upon whether you leave the insulation on the radiating part or not. But what I was concerned about is the transmission line. You lose about half of your power every 15 feet. Until your last posting, you didn't tell us the length of the transmission line but personally I find 1.2 dB matched line loss in six feet to be unacceptable. Any SWR above 1:1 and the losses are even greater. Cecil Moore wrote: I wouldn't use RG58 for anything above HF. It has 20 dB matched line loss per 100 ft at that frequency. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
#13
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In article ,
David wrote: Dave, If I scale that up to 915MHz, the dimensions are almost perfect for using RG58 coax instead of the copper tubing.The outer tube would be 5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I do not need to slide the tap point up as their design did. Bad assumption, I think. The portion of the matching section between the tap point and the bottom is a shorted stub, which adds some inductive reactance at the feedpoint. If you don't have it, you won't get a good match. Another approach I've seen to creating the necessary shorted stub is to just use RG-58 and a simple BNC "T" connector. If you look at the Sperrtopf antenna design, picture it made this way: - one long piece of RG-58 1380 mm long, with the upper part of the outer shield removed... that's your radiator, and the portion of the matching section above the feedpoint. - A second section of RG-58, about 100 mm long, with one end shorted. That's the stub. - The feedline RG-58. Now, just connect all three together... radiator/matching section, stub, and feedline. You can solder 'em, or add BNC plugs and use a BNC "T". If you do the latter, remember to include the lengths of the plugs and "T" into account. I suppose I should also add the choke coil at the antenna base for additional isolation of ground currents. Certainly would not hurt to do so. You could either coil the coax below the bottom of the matching section, or add a quarter-wave choke sleeve, or just run the coax through a few ferrite beads. The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If the bottom part of their design is 505mm then the antenna must be used for 2m band. If this is the case, how come the radiating element is 1480 long ? I think there's some confusion in the antenna schematic diagram, compared with the text and parts list. The diagram appears to show the upper (radiating) portion of the center element being 1480 mm, but the parts list indicates that the *total* length of the inner element is 1480 mm. The latter seems more correct to me... it'd give you a radiating element of (1480 - 505) = 975 mm, which is a bit less than 1/2 physical wavelength at 2 meters. This seems reasonable to me given that the radiating element is fairly thick (10mm) rather than a thin wire. The Coaxial antenna mentioned in the article mentioned by Jerry looks good. The version they mention with a GAP would be simple to construct. It is a 1/4 wave radiating element at the top (earth braid stripped back), then 1/4 wave of full coax, then a "small" gap, then the transmission line. They do not mention the size of the gap but I assume any small gap has the same effect of isolating the reverse current ? I'm not clear about the gap configuration either. The article sort of implies that its size has to be set experimentally, so that the leakage current through the gap is the right amount to cancel out the signal propagating back down the outside of the upper section. The article doesn't say whether the gap is supposed to go around the full circumference of the coax (completely isolating the upper and lower portions of the shield) or only partially around... I infer that it's the latter, because I don't see how the antenna could work with the shield sections fully separated. Should the actual lengths of these sections be modified by "K" depending on diameter of conductor or are they exact 1/4 wave length cuts ? Well, I think it's a bit of a compromise. The upper section (the coax center conductor and its surrounding dielectric) is going to have a velocity factor of not much less than 1.0. Cutting to 1/4 physical wavelength, and then trimming a bit, seems likely to work. I think the lower section is likely to be a bit trickier, since there are actually two velocity factors involved. The radiating is done by the RF travelling back down the outside of the coax braid from the feedpoint, and this (like the upper section) will have a velocity factor close to 1.0, which suggests that the gap-to-feedpoint distance should be close to 1/4 physical wavelength. HOWEVER: the signal travelling up the *inside* of the braid, like that travelling up the inner conductor towards the feedpoint, will be travelling more slowly - the velocity factor will be somewhere around ..66 - .8 depending on cable type. This means that there will be more than 180 degrees of phase delay between the signal passing the gap going upwards (inside the cable) and the signal reaching the gap from above (travelling back down the outside of the cable). Hence, the gap leakage won't result in full cancellation of the signal travelling down the outside of the coax past the gap. To get the two signals into accurate 180-degree phase opposition, you'd need to make the distance from the feedpoint to the gap a bit less than 1/4 physical wavelength, but a bit more than 1/4 of the coax's usual electrical wavelength. Splitting the difference might work fairly well. I'm not sure what this will do to the antenna's feedpoint impedance / return loss, though, and it'll probably tilt the antenna's radiation pattern somewhat. You will probably need to experiment to get the best compromise between antenna pattern and feedpoint impedance / return loss / SWR. The same is true of the gap size and configuration. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#14
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#16
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Suggest you do a google search for baluns coupled with each of the following
Maxwell, Cebik, and Lewellen. You will become enlightened grasshopper... wrote in message ... On Thu, 15 Sep 2005 18:37:48 -0000, (Dave Platt) wrote: = If the antenna feed point is a matched condition what is the RF on the coax shield from? It should be very small. |
#17
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Fred W4JLE wrote:
Suggest you do a google search for baluns coupled with each of the following Maxwell, Cebik, and Lewellen. . . . Searching for Lewallen will probably be more fruitful than Lewellen. Roy Lewallen, W7EL |
#18
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Mea Culpa, my fingers slipped when typing, and I in a senior moment failed
to proof. In any event your piece on baluns is part of my reference library. "Roy Lewallen" wrote in message ... Fred W4JLE wrote: Suggest you do a google search for baluns coupled with each of the following Maxwell, Cebik, and Lewellen. . . . Searching for Lewallen will probably be more fruitful than Lewellen. Roy Lewallen, W7EL |
#19
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Thanks everyone for your feedback to my posting.
I have now been given a lot of information but am starting to go around in circles. It seems many people are used to operating at lower frequencies with huge lengths of wire and hundred of watts of power. In my application, I am looking for an omni-directional antenna with reasonable "gain" and fairly low radiation angle. The application operates with Data at 920 MHz where power levels are below 1 watt. The antenna must either fix directly to the enclosure (which can be plastic or metal and is usually only around 100mm x 60mm x 40mm) or within 2-3m of the enclosure. Therefore, a unit that does not require ground plane style antenna would be good. The end-fed antenna style is good because I can build it into the end of a SMA connector and use 1/4" nylon tubing as the Radome. Now I just need what goes in the Radome to complete the unit. It is difficult to install Tee BNC connectors and mid tap points on coils or transmission lines etc. Stripping coax cable and coiling the coax on the outside of the Radome is fine as I can cover the unit later with heatshrink. In my search it seemed the J-pole was a good contender due to its performance and low radiation angle and ground independence. The shorted stub version was troublesome but the open stub looked promising. It appeared as though I could simply use coaxial cable to create the antenna and fold back part of the coax over 1/4 wave section. Some have suggested coiling cable to act as a choke at the feed point and others have mentioned not to do this. Would anyone by chance have details of a whip antenna that is proven to work and is similar to what I am trying to construct that I could use as a starting point ? Thanks in advance. Roy Lewallen wrote: Fred W4JLE wrote: Suggest you do a google search for baluns coupled with each of the following Maxwell, Cebik, and Lewellen. . . . Searching for Lewallen will probably be more fruitful than Lewellen. Roy Lewallen, W7EL |
#20
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David wrote:
Would anyone by chance have details of a whip antenna that is proven to work and is similar to what I am trying to construct that I could use as a starting point ? In "Antennas For All Applications", by Kraus and Marhefka, third edition, page 824, a 1/2WL over 1/4WL collinear array vertical is described that allegedly has a gain of 6.4 dBi. There is a 180 deg. phase reversing coil between the bottom 1/4WL and the top 1/2WL. The entire vertical should be less than one foot long. The 180 deg. phase reversing coil is the tricky part. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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