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#11
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matching 50 ohm line to 75 ohm antenna
OK, It's a monoband LP. I am assuming that the inter-element crossover connections are correct. Yes, double checked the assembly instructions The feedpoint impedance is determined primarily by the dimensions and spacing of the boom elements. They make a two element open wire transmission line. This is fixed by the design and is not readily tunable [adjustable]. However, secondary tuning adjustments are possible. Its a plug and play, no tune LPA however i feel there are some strage circumstances with the feed or something as it worked fine at a different location & feedline length. The "shorted with a bar" is a tuning stub that provides some degree of adjustment. As stated in another post you may have a 200 ohm or possibly 300 ohm feedpoint impedance [VSWR 4:1]. Please tell us what VSWR value you are seeing. A VSWR of 4:1 indicates a 200 ohms design; 6:1 indicates a 300 ohm design, etc. No its a fixed bar and direct 50 connection. Check the dimensions of the tuning stub for length as described by the manufacturer. No tunning A LP is a balanced driven array. It should NOT be fed directly with coax [as you describe]. Direct feed causes a current imbalance and currents on the outside braid of the coax. You need either a choke balun or a ferrite based 6 meter balun. No balun is needed however there maybe something strange going on so I will try a coax balun and try a fiberglass mounting pole, bottom line is i moved this LPA about 30 foot, changed the polarity from vert to hor, same type of feedline was used but instead of 70' its 35'long otherwize everythiong stayed the same. |
#12
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matching 50 ohm line to 75 ohm antenna
Dave wrote: [snip] A LP is a balanced driven array. It should NOT be fed directly with coax [as you describe]. Direct feed causes a current imbalance and currents on the outside braid of the coax. You need either a choke balun or a ferrite based 6 meter balun. Actually if the coax is fed through one half of the boom with the outer conductor attached to that side and the center conductor connected to the other half of the boom you don't need a balun. |
#13
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matching 50 ohm line to 75 ohm antenna
Tam/WB2TT wrote: "N4aeq" wrote in message oups.com... Dave wrote: N4aeq wrote: Im trying to match 50 ohm coax (lmr400) with a log peroidic which should have a 75 ohm feedpoint the SWR is flat across the 6 m band but high. Im thinking i need a better match but not sure where to start. You have not stated which LP you are using. So, my comments are general in nature. The feedpoint impedance of a LP does vary as a function of frequency and the log characteristics of the elements, their spacing, and the boom geometry. The feedpoint impedance will cycle between a low to high value and generally remain below 1.7:1. Example: my HF LP [Tennadyne T8] can have any feedpoint impedance between 29.4 and 85 ohms as I tune from 13.5 MHz to 30 MHz and it will have numerous excursions within the frequency range. That is the Physics of LP design. Since the feedpoint impedance is a variable it is not easily matched to a standard coax line. Now, a VSWR of 1.7:1 relates to approximately 93% of the rigs power being coupled to the antenna. To your comment. What is the 6 meter VSWR? If it is less than 2:1 [90% coupling to the antenna] then forget it! Im using a double boom LPA, 50 coax connected to the front with shield to one boom and cc to the other. The back of the boom is shorted with a bar, it is a monoband 50-54mhz 5 element. Swr nowhere near 2:1 (i wish) and i have checked the line so its ok so maybe i should have ask how do you adjust the impeadance on a double boom lpa? I still think it is a 200 Ohm antenna. Tam On a double boom LPA ithe nominal value can be around 50 ohms, It depends on the spacing of the booms. Bill Orr has a design for 6 meters that used 3 or 4 elements in an LPA and then added a few parasitic elements. I tried this antenna and mine had no noticeable improvement over my 4 element Yagi Jimmie D |
#14
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matching 50 ohm line to 75 ohm antenna
Tam/WB2TT wrote: "N4aeq" wrote in message oups.com... Dave wrote: N4aeq wrote: Im trying to match 50 ohm coax (lmr400) with a log peroidic which should have a 75 ohm feedpoint the SWR is flat across the 6 m band but high. Im thinking i need a better match but not sure where to start. You have not stated which LP you are using. So, my comments are general in nature. The feedpoint impedance of a LP does vary as a function of frequency and the log characteristics of the elements, their spacing, and the boom geometry. The feedpoint impedance will cycle between a low to high value and generally remain below 1.7:1. Example: my HF LP [Tennadyne T8] can have any feedpoint impedance between 29.4 and 85 ohms as I tune from 13.5 MHz to 30 MHz and it will have numerous excursions within the frequency range. That is the Physics of LP design. Since the feedpoint impedance is a variable it is not easily matched to a standard coax line. Now, a VSWR of 1.7:1 relates to approximately 93% of the rigs power being coupled to the antenna. To your comment. What is the 6 meter VSWR? If it is less than 2:1 [90% coupling to the antenna] then forget it! Im using a double boom LPA, 50 coax connected to the front with shield to one boom and cc to the other. The back of the boom is shorted with a bar, it is a monoband 50-54mhz 5 element. Swr nowhere near 2:1 (i wish) and i have checked the line so its ok so maybe i should have ask how do you adjust the impeadance on a double boom lpa? I still think it is a 200 Ohm antenna. Tam On a double boom LPA ithe nominal value can be around 50 ohms, It depends on the spacing of the booms. Bill Orr has a design for 6 meters that used 3 or 4 elements in an LPA and then added a few parasitic elements. I tried this antenna and mine had no noticeable improvement over my 4 element Yagi Jimmie D |
#15
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matching 50 ohm line to 75 ohm antenna
JIMMIE wrote:
SNIPPED On a double boom LPA ithe nominal value can be around 50 ohms, It depends on the spacing of the booms. Bill Orr has a design for 6 meters that used 3 or 4 elements in an LPA and then added a few parasitic elements. I tried this antenna and mine had no noticeable improvement over my 4 element Yagi Jimmie D Jimmie, I would expect no improvement in GAIN over a four element Yagi. But how does the VSWR compare between the two antennas when measured continuously from 50.0 to 54.0 MHz? I expect the LP will have a more uniform VSWR. That is why people choose LPs. Namely to get a 'flatter' VSWR response. /s/ DD |
#16
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matching 50 ohm line to 75 ohm antenna
"Dave" wrote in message . .. JIMMIE wrote: SNIPPED On a double boom LPA ithe nominal value can be around 50 ohms, It depends on the spacing of the booms. Bill Orr has a design for 6 meters that used 3 or 4 elements in an LPA and then added a few parasitic elements. I tried this antenna and mine had no noticeable improvement over my 4 element Yagi Jimmie D Jimmie, I would expect no improvement in GAIN over a four element Yagi. But how does the VSWR compare between the two antennas when measured continuously from 50.0 to 54.0 MHz? I expect the LP will have a more uniform VSWR. That is why people choose LPs. Namely to get a 'flatter' VSWR response. /s/ DD I was never really interested in covering the whole band , just the first 1.5 Mhz or so. The SWR was comparable in this area on both antennas. The Yagi was easier and cheaper to build. |
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