[Antennas] Re: increased ant bandwidths...

[Robert Lay (W9DMK)]

Dear David,

There are some graphics in the ARRL Antenna Book that are pretty 
straightforward to use. They give the dB of attenuation per 100 ft of almost 
any type of coax you will find in use today. All one has to do is determine 
the length of coax and its type, make an entry to those curves for any given 
frequency and the loss comes out as a simple loss in dB. Each 3 dB of loss 
is a 50% reduction in power. It's that simple.

73 de W9DMK - Bob Lay, Dahlgren, VA
http://www.zaffora.com/W9DMK/W9dmk.html
w9dmk at crosslink.net
----- Original Message ----- 
From: "DavidE Benedict" <iam at pmug.org>
To: <223bthp at cox.net>; <antennas at mailman.qth.net>
Sent: Saturday, April 29, 2006 3:53 PM
Subject: Re: RE: [Antennas] Re: increased ant bandwidths...


> 223bthp at cox.net writes:
>>Bob and Roy,
>>
>>It sounds like you both agree that power output from the transmitter is
>>the
>>only objective of any antenna tuner. What happens after that we have to
>>accept if we want wider use of each antenna.
>>
>>If antenna tuners were placed at the feedpoint of the antenna, there would
>>be little reflected power on the transmission line before it. An antenna
>>coupler is always as close to the feedpoint as possible for this reason.
>>
>>When we want more tuning options in non-weatherproof assemblies right at
>>our
>>fingertips (and inside the zone of our lightning protection system), we
>>have
>>to accept the consequences. An inside antenna tuner still protects the
>>transmitter and the operator, as long as we remember the output of the
>>tuner
>>(like a coupler) is 'hot' in more ways than temperature.
>>
>>Regards,
>>
>>Jack
>
>
> What's realistic, though, for input and output power?
>
> Lets face it, the major SWR loss in a tuned antenna system is loss in the
> coax, ...and, typically only a few percent in a quality tuner.
>
> So, a huge SWR on lo-loss ladder line, or in using a tuner next to an
> antenna feed-point, is almost negligible compared to a huge SWR between
> the tuner and antenna using 50 feet of RG-58.
>
> So, one needs to get a short transmission line which is lo-loss, et
> vipola...mucho power out.
>
> But, how much is the difference? Is it worthwhile to do some of the things
> this indicates would be a good idea? How can we know?
>
> For instance,...
> If I put out 100 watts on 40 meters into a 10M dipole, tune it with a
> tuner to 1:1, how much do I actually radiate when using 50 feet of RG-8?
> -- Then what happens if I switch the parameters to 100 W on 10m using a
> 40m dipole?
> -- Then what happens if I increase the coax length 10, then 20, then 30
> more feet?
> -- It seems like a couple pages of line-item data something like this
> would give a lot of wisdom on the Tuner/ SWR subject.
>
> Anyway, there is such a mystique about SWR, tuners, etc.
> (I am whining there).
>
> So, how about using rules-of-thumb & nomographs (sp?)?
> Everyone uses them, probably even the nuclear guys. So, why don't we have
> simple (leave out all the not-so-power-robbing factors), and have,
> generally, a lot more savvy within the Amatuer community about antennas &
> tuners, and IN & OUT power differences which simple things aid or cause to
> be worse?
>
> I don't have a dog in this competition because I operate only mobile and I
> have a Larry's Screwdriver, 160m-6m, and do quite well, so my problems are
> pretty much solved with this
> "dummy-load-with-a-capacity-hat-and-a-tuned-inductor". Maybe I am
> unrealistic but I'd like to see a lot less mystique about these antenna
> things.
>
> David B.
> W7DBH
> Mobile-Only ...in NW Oregon
>
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