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matching masts as vertical antennas.
"Owen Duffy" wrote in message ... "Wayne" wrote in news:srSpi.4065$9A6.827@trnddc01: "Owen Duffy" wrote in message ... "Wayne" wrote in news:cbKpi.3766$7w.1151@trnddc05: ... You might try the matching system used on the old "Gotham Vertical". This uses a tapped coil from the base of the mast to ground. The top of the coil is tapped down until resonance is found. Coax braid is to ground, and the coax center conductor is tapped up from the bottom of the coil for best match. Whilst this system might work for an antenna with a capacitive feedpoint, will it work for an inductive feedpoint such as this ~3/8 lambda attena? Owen Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't that work? I don't really understand exactly what you mean, though I know it is a popular proposition. If the antenna feedpoint is around 100+j300 (see my previous posting), the simplest matching network is an L network, and there are two solutions to it, but both L networks in this specific case require an L and a C, and both use practical values of L and C. For example, a 84pf capacitor in shunt with the feedpoint, then a 2.4uH inductor in series from the feedline to the feedpoint will deliver that transformation with reasonable efficiency and with one side of the capacitor grounded. Can you propose the values of a two inductor L network, or a tapped inductor that will transform 100+j300 to 50+j0. Owen I spent some time pondering this with EZNEC and a smith chart. No, I can't come up with an inductive way to match it. I learned somthing today...thanks! |
matching masts as vertical antennas.
On Jul 26, 11:55 am, "Wayne" wrote:
I spent some time pondering this with EZNEC and a smith chart. No, I can't come up with an inductive way to match it. I learned somthing today...thanks! A 3/8 wave radiator is easily matched with a single variable cap.. Or should be anyway... MK |
matching masts as vertical antennas.
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matching masts as vertical antennas.
"Wayne" wrote in
news:Ll4qi.15820$U47.4532@trnddc08: .... Owen I spent some time pondering this with EZNEC and a smith chart. No, I can't come up with an inductive way to match it. I learned somthing today...thanks! The components for a L matching network may both be L for some range of loads, but not in the case discussed. There will also be loads where only a shunt component is required, and this is often achieved on a short vertical by using less inductive loading than needed to bring the feedpoint to a purely resistive load. You can think of it as an L network where the series element is supplied by the antenna. For example, a short antenna on 3.5MHz with a feedpoint resistance of say 20 ohms, and a large capacitive reactance can be inductively loaded to deliver 20-j24.5, and it will require only a shunt inductor of about 0.9uH to match it to 50 ohms. (Neither of these are solutions to the original questions, but interesting sidepaths that show why some configurations work in some situations.) Owen |
matching masts as vertical antennas.
On Jul 26, 5:14 pm, Owen Duffy wrote:
wrote roups.com: A 3/8 wave radiator is easily matched with a single variable cap.. Details? Place cap at the feedpoint in series . Viola... This is fairly common really.. IE: feeding "extended" Inv L's where the length is appx 3/8 wave. The usual reason for that is to raise the maximum current point up off the ground a bit to lower ground losses.. BTW, once you find the best match, and find the cap value needed, you could replace the variable with a fixed cap. MK |
matching masts as vertical antennas.
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matching masts as vertical antennas.
Owen Duffy wrote:
cliff wright wrote in : wrote: On Jul 26, 5:14 pm, Owen Duffy wrote: wrote glegroups.com: A 3/8 wave radiator is easily matched with a single variable cap.. Details? Place cap at the feedpoint in series . Viola... This is fairly common really.. IE: feeding "extended" Inv L's where the length is appx 3/8 wave. The usual reason for that is to raise the maximum current point up off the ground a bit to lower ground losses.. BTW, once you find the best match, and find the cap value needed, you could replace the variable with a fixed cap. MK G'Day! Well I have tried the series cap idea of course first. It is set up at the moment with a 500 pF TX type variable in the matching box. However according to my SWR bridge it has quite a high VSWR and varying the 500 pF had very little effect. And yes, the coax is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK. My phased verticals that I mentioned did use capacitive matching in this case 1000 pF RX variables. But they were just a little too long. Unfortunately because of space restrictions I am limited to 4 radials. Thats one reason why I was trying for more than 1/4 wave. 73's Cliff wright ZL1BDA. Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the limitations to his method. The proposed series capacitor will only achieve a reasonable match if the series resistance component of the feedpoint is around 50 ohms, and the reactance is inductive and within range of a practical capacitor. If indeed the antenna had a feedpoint impedance around 100+j300, the series capacitor would not achieve a match better than VSWR~=2 (which may or may not be good enough). Your observation that it was bad (whatever "quite a high VSWR" means), suggests that it has a higher feedpoint R (it is unlikely to be lower than 50 ohms), or that your capacitor did not tune out the inductive reactance (unlikely since you used a 500pF variable). If you can't match it with a series cap (and no surprises for me there), you could always try an L network starting with the values that I previously gave you. Owen Yes Owen I reckon you are right! Tried it out again today and the VSWR is a bit worse than 2:1, not good enough for me at all. Now all I need is a good reference work to brush up my complex impeadnaces that I haven't looked at for about 30 years! 73's ZL1BDA |
matching masts as vertical antennas.
cliff wright wrote:
Owen Duffy wrote: cliff wright wrote in : wrote: On Jul 26, 5:14 pm, Owen Duffy wrote: wrote roups.com: A 3/8 wave radiator is easily matched with a single variable cap.. Details? Place cap at the feedpoint in series . Viola... This is fairly common really.. IE: feeding "extended" Inv L's where the length is appx 3/8 wave. The usual reason for that is to raise the maximum current point up off the ground a bit to lower ground losses.. BTW, once you find the best match, and find the cap value needed, you could replace the variable with a fixed cap. MK G'Day! Well I have tried the series cap idea of course first. It is set up at the moment with a 500 pF TX type variable in the matching box. However according to my SWR bridge it has quite a high VSWR and varying the 500 pF had very little effect. And yes, the coax is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK. My phased verticals that I mentioned did use capacitive matching in this case 1000 pF RX variables. But they were just a little too long. Unfortunately because of space restrictions I am limited to 4 radials. Thats one reason why I was trying for more than 1/4 wave. 73's Cliff wright ZL1BDA. Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the limitations to his method. The proposed series capacitor will only achieve a reasonable match if the series resistance component of the feedpoint is around 50 ohms, and the reactance is inductive and within range of a practical capacitor. If indeed the antenna had a feedpoint impedance around 100+j300, the series capacitor would not achieve a match better than VSWR~=2 (which may or may not be good enough). Your observation that it was bad (whatever "quite a high VSWR" means), suggests that it has a higher feedpoint R (it is unlikely to be lower than 50 ohms), or that your capacitor did not tune out the inductive reactance (unlikely since you used a 500pF variable). If you can't match it with a series cap (and no surprises for me there), you could always try an L network starting with the values that I previously gave you. Owen Yes Owen I reckon you are right! Tried it out again today and the VSWR is a bit worse than 2:1, not good enough for me at all. Now all I need is a good reference work to brush up my complex impeadnaces that I haven't looked at for about 30 years! 73's ZL1BDA Owen I just happened across a reference on page 248 of "Amateur radio Techniques" by Pat Hawker which I had missed before. LUckily I have a good stock of "minductor" type coils and hope sometime next week to try the arrangement at(d) in figure 64. I have also got a stock of miniductor tapping clamps so it will be easy to vary the feed point and tuning connections. With a bit of luck, and some good weather I might have it all set up by about Friday next. Many thanks for your suggestions. 73"s Cliff Wright ZL1BDA ex G3NIA. |
matching masts as vertical antennas.
On Jul 27, 8:21 pm, Owen Duffy wrote:
cliff wright wrote : wrote: On Jul 26, 5:14 pm, Owen Duffy wrote: wrote legroups.com: A 3/8 wave radiator is easily matched with a single variable cap.. Details? Place cap at the feedpoint in series . Viola... This is fairly common really.. IE: feeding "extended" Inv L's where the length is appx 3/8 wave. The usual reason for that is to raise the maximum current point up off the ground a bit to lower ground losses.. BTW, once you find the best match, and find the cap value needed, you could replace the variable with a fixed cap. MK G'Day! Well I have tried the series cap idea of course first. It is set up at the moment with a 500 pF TX type variable in the matching box. However according to my SWR bridge it has quite a high VSWR and varying the 500 pF had very little effect. And yes, the coax is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK. My phased verticals that I mentioned did use capacitive matching in this case 1000 pF RX variables. But they were just a little too long. Unfortunately because of space restrictions I am limited to 4 radials. Thats one reason why I was trying for more than 1/4 wave. 73's Cliff wright ZL1BDA. Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the limitations to his method. I wasn't really expecting the feed R to be that high.. If it's 100 ohms, then yep, the best match would be 2:1. If that's the case, then yes, he would need to use an L network if he wants the match better than that. I never had any trouble using this method, but thinking about it, most of mine were with 160m inv L's.. Maybe it was the shorter vertical section that gave me a low enough feed Z to match . I'd have to model it to see. MK |
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