From: Gavin Stephens

To: LPFM_Radio@...

Sent: Friday, February 15, 2008 3:10 PM

Subject: Re: [LPFM] Re:Reducing output signal

Unfortunately it's not as simple as working it out to a mW per meter. That's why things are done in dB.

 

For example, driving a coax cable with a known loss of say 3dB for a given frequency with a 500mW TX, means 250mW is possible through the cable.  Since 3dB loss (-3dB) would be the RF output power divided by two. If it was gain like a 3dB antenna then it's the opposite, times 2.

 

Driving it with a 1 Watt transmitter /2 however means 500mW will be possible depending on antenna characteristics etc...

 

6dB of loss on the other hand would mean the available signal through the same cable type, freq used etc... is four times more loss.

 

Here's some calculators for RF stuff in html format. Some might find them useful to have up their sleeve. One's a cable loss one the other working out power budgets. They're just one's I've saved off the net for my own personal use.

 

RG-58A's loss at 88.4MHz is 13.617dB per 100 meters. Ewwww (yes good for wacking back power).

 

 

I like RG-213 for LPFM and Altelicon CA-400 for microwave use.

 

Cheers,

Gavin.

 

 

 

 

----- Original Message -----

From: Michael Rowse

To: LPFM_Radio@yahoogroups.com.au

Sent: Friday, February 15, 2008 5:56 AM

Subject: Re: [LPFM] Re:Reducing output signal

Hey Johnno,

 

Studying your figures, it seems that RG58A has a loss of 13mW per metre? So would I be right in saying to get a 1 watt TX down to approx. 300mW or less I would need around 60 metres of RG58A?

 

Cheers, Mike

 

 

On 15/02/2008 12:01:31 a.m., Johnno (valentine@xtra.co.nz) wrote:
> Hello Grant,
>
> Will be a little loss in TX power... now to get my calculator out :)))
>
> If your TX is out putting 500mw @ 88.4 MHZ, SWR of 1:1 at 7 meters of
> coax
> will be
>
> RG58 = Output 409 mw
> RG58A = Output 401 mw
> RG58C = Output 384 mw
> RG213 = Output 450 mw
>
> If you have already increased TX to 1 watt for the 87.5 - 88.4 as RSM is
> talking about then the following applies..
>
> RG58 = Output 818 mw
> RG58A = Output 803 mw
> RG58C = Output 768 mw
> RG213 = Output 901 mw
>
> I hope that helps...
>
> Johnno
>
> ----- Original Message -----
> From: "88.4 FM The Cheese" <studio@thecheese.co.nz [link:
> mailto:studio@thecheese.co.nz]>
> To: <LPFM_Radio@yahoogroups.com.au [link: mailto:LPFM_Radio@yahoogroups.
> com.au]>
> Sent: Thursday, February 14, 2008 10:46 PM
> Subject: [LPFM] Re:Reducing output signal
>
> > This is of interest to me, as I need to move my transmitter for a few
> > weeks while decorating the studio.
> > I have a 7 metre coax extension lead. Will this cause much loss to my
> > transmitter power?
> >
> > Cheers,
> > Grant
> >
> >
> >

---

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