[Antennas] J-Pole

Fri Oct 1 08:26:57 EDT 2004

Wasn't the original premise that the antenna was being fed with 
open-wire line?
My question again is ... how do you measure SWR AT the antenna other 
than how I have suggested?
Don
N8DE
John Tait wrote:
>>>>> John,
>> Are you suggesting that THREE baluns would make the measurement possible?
>>>> A- --------------------
>> N |4:1 balun|-swr meter-|1:4 balun|[open wire feedline]|4:1 balun|-coax
>> T- --------------------
>>>> Hope that diagram looks ok...
>> Antenna connected to 4:1 balun connected to coax with 50-ohm swr meter 
>> connected to 1:4 balun connected to open-wire feedline connected to 
>> 4:1 balun connected to coax ... to rf source.
>>>> I can't see ANY other way of measuring feedline LOSS AT THE ANTENNA.
>>>> No.. I'm saying coax from rig to matching unit, to open wire line, to 
> matching unit, to coax to antenna.. The matching unit is not 
> neccessarily a 4:1 balun..
>> John EI7BA..
>>>>> Don
>> N8DE
>>>> John Tait wrote:
>>>>>>>> Subject: Re: [Antennas] J-Pole
>>>>>>>>>> John...
>>>>>>>> We knew all of that... But anything other than coax is impractical 
>>>> from a
>>>> measurement standpoint...
>>>>>>>>>>>> I don't see why??? you need to measure power in from the TX, and the 
>>> power to the antenna at the other.. The difference between the two 
>>> tells us the efficiency of the line.
>>>>>>> How much test gear exists that will provide
>>>> reasonable accuracy with ladder-line..? With the exception of ancient
>>>> LECHER WIRES, I can't think of any...(smile)
>>>>>>>>>>>> The same gear that you use for your 50 ohm coax will suffice. If 
>>> you're feeding an antenna that was built to be fed by coax, you're 
>>> coming from a 50 Ohm source to a 50 Ohm load via two transformers, 
>>> ATUs or WHY, with open wire line between them. You could measure 
>>> efficiency in the same way as you would a 4:1 balun ..i.e.two baluns 
>>> back to back, measure power in and power out .
>>> e.g 100W in.. 80W out. 20W lost, so 10W lost in each balun so the 
>>> baluns are 90% efficient. (at that frequency). Same procedure with 
>>> your open wire line.
>>>>>> 73
>>> John EI7BA
>>>>>>>>>>>>>> 73, Larry - W1GOR
>>>>>>>>>>>> ----- Original Message -----
>>>>>>>>> From: "John Tait" <bravo at iol.ie>
>>>>>>>>>>>> To: "Joe" <nss at mwt.net>; "W1GOR" <W1GOR at maine.rr.com>
>>>> Cc: <antennas at mailman.qth.net>; "Durwydd MacTara" <durwydd at hotmail.com>
>>>> Sent: Wednesday, September 29, 2004 1:18 PM
>>>> Subject: Re: [Antennas] J-Pole
>>>>>>>>>>>>> No.. I don't think he's kidding.. The whole point of open wire 
>>>>> feeeder is
>>>>> that it has very low loss. It is the most efficient feeder type that's
>>>>> available to us. Line loss increases with frequency, with SWR,and with
>>>>>>>>>>>> line
>>>>>>>>> length, so VHF/UHF antenna system have the most to gain by using 
>>>>> open wire
>>>>> line. Good quality "Ladderline" is a less efficient feeder than proper
>>>>>>>>>>>> open
>>>>>>>>> wire line, but has less loss than coax..'specially if SWR is high.
>>>>> However.. coax is much simpler to deploy than open wire line, 
>>>>> and good
>>>>> quality hardline has acceptable losses so long as SWR is low, and 
>>>>> lengths
>>>>> reasonable. Most of us settle for that, and put up with the losses. 
>>>>> We can
>>>>> compensate for loss of transmitted power by using a linear 
>>>>> amplifier, and
>>>>> for RX loss with a good low noise mast head preamp.
>>>>> 73
>>>>> John EI7BA http://www.iol.ie/~bravo/
>>>>>>>>>>>>>>> >
>>>>> > W1GOR wrote:
>>>>> >
>>>>> >> Ladder-Line for VHF/UHF...? You've gotta be kidding..!
>>>>> >>
>>>>> >> 73, Larry - W1GOR
>>>>> >>
>>>>> >> ----- Original Message -----
>>>>> >> From: "Durwydd MacTara" <durwydd at hotmail.com>
>>>>> >> To: <Antennas at mailman.qth.net>
>>>>> >> Sent: Tuesday, September 28, 2004 10:20 AM
>>>>> >> Subject: [Antennas] J-Pole
>>>>> >>
>>>>> >> > I have a homebrew 2M/70CM J-pole 50 ft. in the air for use with
>>>>> a 5W
>>>>> >> > HT.
>>>>> >> Am
>>>>> >> > currently using RG-213 for feedline. i am wondering if it is 
>>>>> worth
>>>>>>>>>>>> the
>>>>>>>>> >> > effort to switch the feed to ladder line, and if so how to do it?
>>>>> >> >
>>>>> >> > "Carpe` Diem!"
>>>>> >> > Durwydd MacTara
>>>>> >> >
>>>>>>>