[Uram-rejections] Auto ATUs

Wed Aug 16 10:50:14 BST 2017

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============================================ Full text of your message follows
> From webstump at chiark.greenend.org.uk Wed Aug 16 10:23:57 2017
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> From: "Jimbo" <kinvig.netta at ntlworld.com>
> Newsgroups: uk.radio.amateur.moderated
> Subject: Re: Auto ATUs
> Date: Wed, 16 Aug 2017 10:23:22 +0100
> Organization: A noiseless patient Spider
> Message-ID: <on12md$57p$1 at dont-email.me>
> References: <n9e0pc90vdum9cv58q25vmqnl2h0ft8qml at 4ax.com> <omvmnh$l8q$1 at dont-email.me> <on0v8s$r7o$1 at dont-email.me>
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> 
> "Brian Reay" <no.sp at m.com> wrote in message 
> news:on0v8s$r7o$1 at dont-email.me...
> > On 15/08/2017 21:53, Jimbo wrote:
> >> "Guy G4DWV 4X1LT" <no-email at tardis.com> wrote in message
> >> news:n9e0pc90vdum9cv58q25vmqnl2h0ft8qml at 4ax.com...
> >>> I will be buying an auto ATU and am torn between an LDG or MFJ unit.
> >>>
> >>> The MFJ seems to be much better value for money, but I have heard
> >>> about the MFJ build quality and what the letters MFJ have come to
> >>> mean.
> >>>
> >> make a resonant antenna auto atu's are for the lazy ........
> >>
> >>
> >
> >
> >
> >
> > ATUs compensate for antenna, or antenna feed system, impedance, not 
> > resonance. You can achieve a perfect 'match' into an antenna, or load, 
> > which is neither resonant nor a good radiator.
> >
> > For example, a dummy load isn't resonant but doesn't require and ATU. 
> > Likewise, a long length of coax, even with nothing at the 'far end', will 
> > present a good match. It isn't resonant.
> >
> > Likewise, if you make, a resonant antenna but feed it at the wrong point, 
> > you will need an ATU. People tend to think that antennas have a specific 
> > feed point but they don't. The 'normal' feed point is generally chosen for 
> > a mix of convenience- including matching to the feeded. You will have 
> > heard of an off centre fed dipole- the off centre bit adjusts the feed 
> > impedance.
> >
> > Likewise, in the old 'Slim Jim', the basic resonant frequency is set by 
> > the overall dimensions- the length. Sliding the feed point doesn't change 
> > the resonant frequency, it adjusts the impedance point the coax 'taps 
> > into' on the antenna.
> >
> >
> > It is a common error to think ATUs 'tune' antennas for resonance, they 
> > don't. They simply act as impedance 'transformers', by introducing 
> > reactances, to make the antenna system 'look like' the ideal load- 
> > normally 50 ohms these days.  Depending on their location, they may not 
> > even reduce SWR or losses due to SWR.
> >
> > If you have you ATU on your desk, with a long coax run to a mismatched 
> > antenna, the ATU may well make things look dandy but the SWR in the long 
> > feed run is still high as are the losses.
> >
> > Stick the ATU next to the antenna and things are far better, the long 
> > cable run is on the matched side, and the unmatched side is short.
> >
> >
> >
> >
> >
> >
> I agree with all that........but they are still for the lazy ... 
> 
> 
> 
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