[Amps] Asynchronous coax matching transformer
Larry Benko
xxw0qe at comcast.net
Sat Aug 1 18:34:55 PDT 2009
Roger,
You are confusing insertion loss with return loss. A return loss of
20dB is the same as an SWR of 1.22:1 which is also equal to a reflection
coefficient of 0.1. Insertion loss is a combination of dissipative
(heat producing) loss and mismatched loss.
73,
Larry, W0QE
Roger wrote:
> Carl wrote:
>
>> UHF connectors have varying effects but for all pratical purposes show a
>> 20dB or better return loss thru 222 mHz before climbing. Many versions do
>>
>>
> 20 db? I've strung a whole series of connectors together and didn't get
> half a db. 275 feet of coax (LMR 400) with connectors at the rig to
> grounding panel/Polyphaser(2), cable to base of tower (2), bulkhead
> connector, cable to top of tower (2), grounded bulkhead connector,
> pigtail to 2-way power divider(2), two cables from power divider to
> antennas (2). That's 12 connectors 15 if I count the so239s on the rig
> and antennas. The power reaching the antennas was only a few tenths of
> a db less than that much cable without connectors.
>
>> show RF heating due to poor quality insulation, even at HF.
>>
>> I refuse to buy imports and search out Amphenol, Kings, and other mil spec
>> marked ones at hamfests, sometimes attached to a piece of junk coax.
>>
>>
>>
> I never use, used connectors or even reuse them except for the expensive
> ones. Yes I do occasionally use imports, but I check the quality. If I
> get a batch that aren't up to par I pitch them rather than give them
> away and have some one run into problems because they were trying to
> save money. That was one of the reasons I went to crimp connectors.
>
> 73
>
> Roger (K8RI)
>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message -----
>> From: "Joe Subich, W4TV" <lists at subich.com>
>> To: <TexasRF at aol.com>; <gm3sek at ifwtech.co.uk>; <amps at contesting.com>
>> Sent: Saturday, August 01, 2009 4:35 PM
>> Subject: Re: [Amps] Asynchronous coax matching transformer
>>
>>
>>
>>
>>>
>>>
>>>> If the sections are joined via coax connectors, you have to
>>>> take into account where 75 ohms ends and 50 ohms begins. The
>>>> connectors are invariably of a 50 ohm type and their internal
>>>> length must be added to the 50 ohm part of the matching section.
>>>>
>>>>
>>> It depends on the type of connector. UHF connectors are often
>>> closer to 35 Ohms if I recall correctly. N connectors can be
>>> either 50 or 75 Ohms ... if using a barrel for splicing, it is
>>> probably better to use a 75 Ohm barrel with a 75 Ohm center pin
>>> on the 50 Ohm cable (removing one stand of the center conductor
>>> if necessary). If using a male/female connector pair then one
>>> would use 50 Ohm connectors and "build up" the center conductor
>>> of the 75 Ohm cable.
>>>
>>> For HF work - even through six meters - the connector length
>>> is less than 1 degree and can probably be ignored.
>>>
>>> 73,
>>>
>>> ... Joe, W4TV
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>> -----Original Message-----
>>>> From: amps-bounces at contesting.com
>>>> [mailto:amps-bounces at contesting.com] On Behalf Of TexasRF at aol.com
>>>> Sent: Saturday, August 01, 2009 1:38 PM
>>>> To: xxw0qe at comcast.net; gm3sek at ifwtech.co.uk
>>>> Cc: amps at contesting.com
>>>> Subject: Re: [Amps] Asynchronous coax matching transformer
>>>>
>>>>
>>>>
>>>> Hi all,in addition to the examples Larry offered, the scheme
>>>> will also
>>>> match impedances less than 50 ohms, down to 22.2 ohms if I
>>>> remember correctly.
>>>>
>>>> A two way power divider where two 50 ohm loads are in
>>>> parallel requires a
>>>> 25 to 50 ohm transformer which is easily handled with the
>>>> series matching
>>>> method.
>>>>
>>>> The only down side is the difficulty in adjusting the
>>>> matching sections. On
>>>> the other hand, transmission line calculations via Smith
>>>> Chart or computer
>>>> program will give the exact lengths in degrees needed so
>>>> tuning is not a
>>>> real requirement anyway.
>>>>
>>>> If the sections are joined via coax connectors, you have to
>>>> take into
>>>> account where 75 ohms ends and 50 ohms begins. The connectors
>>>> are invariably of
>>>> a 50 ohm type and their internal length must be added to the
>>>> 50 ohm part of
>>>> the matching section.
>>>>
>>>> The last issue is velocity factor of the coax and connectors.
>>>> A serious
>>>> builder will be prepared to confirm these quantities before
>>>> cutting the
>>>> matching sections to length.
>>>>
>>>> It is a real joy to go through the numbers and have the
>>>> transformer turn
>>>> out right on frequency.
>>>>
>>>> 73,
>>>> Gerald K5GW
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> In a message dated 8/1/2009 10:59:13 A.M. Central Daylight Time,
>>>> xxw0qe at comcast.net writes:
>>>>
>>>> Hi Ian & Carl,
>>>>
>>>> Actually given only 50 ohm and 75 ohm coax and not
>>>> paralleling any coax
>>>> you can match all resistive impedances between 50 and 112.5
>>>> (75*75/50)
>>>> ohms.
>>>>
>>>> e.g.
>>>> 50 ohms ---- 29.3 deg. 75 ohm coax ----- 29.3 deg 50 ohm
>>>> coax ----- 75
>>>> ohms (as shown by the 1/12 wave match)
>>>> 50 ohms ---- 40.5 deg. 75 ohm coax ----- 23.8 deg 50 ohm coax
>>>> ----- 85
>>>> ohms
>>>> 50 ohms ---- 58.3 deg. 75 ohm coax ----- 14.9 deg 50 ohm
>>>> coax ----- 100
>>>> ohms
>>>> 50 ohms ---- 90.0 deg. 75 ohm coax ----- 0.0 deg 50 ohm coax -----
>>>> 112.5 ohms (std 1/4 wave match)
>>>>
>>>> Also many reactive impedances are also easily matched
>>>> e.g.
>>>> 50 ohms ---- 42.6 deg. 75 ohm coax ----- 10.3 deg 50 ohm
>>>> coax ----- 75 -
>>>> j20 ohms
>>>>
>>>> For a matching over a small range of frequencies
>>>> transmission lines work
>>>> well.
>>>>
>>>> 73,
>>>> Larry, W0QE
>>>>
>>>>
>>>> Ian White GM3SEK wrote:
>>>>
>>>>
>>>>> Carl wrote:
>>>>>
>>>>>
>>>>>
>>>>>> Does anyone remember the formula for matching 50 to 75
>>>>>>
>>>>>>
>>>> Ohms with 2
>>>>
>>>>
>>>>>> very short lengths of coax? If I remember it used a 75
>>>>>>
>>>>>>
>>>> Ohm section
>>>>
>>>>
>>>>>> at the output of the amp, and a
>>>>>>
>>>>>>
>>>> 50
>>>>
>>>>
>>>>>> Ohm section to the CATV hardline. Frequency dependent of course.
>>>>>>
>>>>>> Its been about 25 years since I built one.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>> http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm
>>>>>
>>>>> By paralleling two lengths of 50 or 75 ohm line, it can
>>>>>
>>>>>
>>>> also match to
>>>>
>>>>
>>>>> 25
>>>>> or 37.5 ohms; or to even lower impedances by paralleling
>>>>>
>>>>>
>>>> three lengths
>>>>
>>>>
>>>>> of the same line.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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