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#1




matching 50 ohm line to 75 ohm antenna
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. 
#2




matching 50 ohm line to 75 ohm antenna
"N4aeq" wrote in message ps.com... 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. I can't remember the numbers, but there is a procedure in either the ARRL Handbook, or antenna book for doing this. Basically, you have a short piece of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length of 50 Ohm transmission line. If the antenna is 75 Ohms, your SWR on the 50 Ohm line should be no more than 1.5:1. Are you sure you don't need a 4:1 balun? If your SWR is around 3 or 4, I would try the balun; easy neough to make out of a 1/2 wave piece of coax. Tam/WB2TT 
#3




matching 50 ohm line to 75 ohm antenna
Tam/WB2TT wrote:
I can't remember the numbers, but there is a procedure in either the ARRL Handbook, or antenna book for doing this. Basically, you have a short piece of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length of 50 Ohm transmission line. You are thinking of a nonsynchronous balun described he http://www.k1ttt.net/technote/matching.html#k1xx HOWEVER, this only works for one band (i.e. a monobander...not a logperiodic covering multiple bands). If the antenna is 75 ohms, why not simply use 75 ohm coax to the shack and then a tuner to convert to 50 ohms? Seems much simpler and you can normally get free 75 ohm hardline for the asking at your local cable company (i.e. reel ends). 73, Bill W4ZV 
#4




matching 50 ohm line to 75 ohm antenna
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! 
#5




matching 50 ohm line to 75 ohm antenna
Tam/WB2TT wrote:
I can't remember the numbers, but there is a procedure in either the ARRL Handbook, or antenna book for doing this. Basically, you have a short piece of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length of 50 Ohm transmission line. 1/4WL of RG62 followed by 1/4WL of RG59 will give a good match from 75 ohms to 50 ohms.  73, Cecil http://www.w5dxp.com 
#6




matching 50 ohm line to 75 ohm antenna
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 5054mhz 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? 
#7




matching 50 ohm line to 75 ohm antenna
wrote in message oups.com... Tam/WB2TT wrote: I can't remember the numbers, but there is a procedure in either the ARRL Handbook, or antenna book for doing this. Basically, you have a short piece of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length of 50 Ohm transmission line. You are thinking of a nonsynchronous balun described he http://www.k1ttt.net/technote/matching.html#k1xx HOWEVER, this only works for one band (i.e. a monobander...not a logperiodic covering multiple bands). If the antenna is 75 ohms, why not simply use 75 ohm coax to the shack and then a tuner to convert to 50 ohms? Seems much simpler and you can normally get free 75 ohm hardline for the asking at your local cable company (i.e. reel ends). 73, Bill W4ZV He was talking about 6 meters. Tam 
#8




matching 50 ohm line to 75 ohm antenna
On 15 Nov 2006 12:03:12 0800, "N4aeq" wrote:
You have not stated which LP you are using. Hi OM, You never really answered this for Dave. Im using a double boom LPA, 50 coax connected to the front with shield to one boom and cc to the other. Do you have any feedline decoupling/choking? The back of the boom is shorted with a bar, That should be a hairpin matching element. it is a monoband 5054mhz 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? The point of a Log Periodic is you are not expected to tune it. The tune is built into the design as long as it is up high enough (and probably even if it isn't). If you SWR is so wildly off, you have a wild problem (feedline is undecoupled is my guess). 73's Richard Clark, KB7QHC 
#9




matching 50 ohm line to 75 ohm antenna
"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 5054mhz 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 
#10




matching 50 ohm line to 75 ohm antenna
N4aeq wrote:
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 5054mhz 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? OK, It's a monoband LP. I am assuming that the interelement crossover connections are correct. 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. 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. Check the dimensions of the tuning stub for length as described by the manufacturer. 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. 
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