Re: [FT897] SWR Problem
- Sometimes it can be the feedline length, but I am suspecting the coax
connector. Barrel connectors will add in some reactance, but usually not
enough to make a huge difference.
On Wed, May 8, 2013 at 10:02 AM, jamespgrove <james@...>wrote:
> This maybe a somewhat basic question but i am struggling to work out why i
> get higher SWR readings when i extend my feed line to my radio.
> I have a Diamond Antenna mounted on a pole on the side of my house, i have
> around 10m of RG8 Mini cable that feeds into the shack where my radio is.
> This works fine, SWR is flat as it should be.
> I want to use my radio in the other room so i join (using a joiner)
> another feed line to the existing one, the same cable same specs, i also
> add another using the same method, so total cable length is under 20m.
> However i get some higher level of SWR than i do if the radio is connected
> in my shack. So the additional 10m of RG8 Mini is causing the higher SWR.
> Can anyone point me in the direction of the problem? I have some ideas but
> nothing short of minor guess work, ie the cable length a loss?
> The SWR is not a huge problem, as its not dangerously high i was just
> expecting it to be flat still.
> Any help appreciated,
> Jim M6BXG
[Non-text portions of this message have been removed]
- I think that you missed the point of what was being said. If the load is
not matched to the system impedance then the complex impedance seen at the
transmitter *will* vary with feeder length, BUT the *actual impedance of the
load* will only be seen at *at the transmitter* when the feeder length is
1/2 wavelength or a multiple. That of course assumes lossless coax; where
there is loss the impedance seen at the tx will be modified by the loss only
and not the electrical length at the 180 degree points. The phase of the
mismatch *will* change with coax length, but it will repeat every 1/2
The swr seen at the Tx will not vary due to electrical length, just due any
additional loss in the coax.
From: FT897@yahoogroups.com [mailto:FT897@yahoogroups.com] On Behalf Of
Sent: 10 May 2013 15:38
Subject: [FT897] Re: SWR Problem
This statement is not entirely correct.
If your antenna impedance is 50 ohms and the feedline has a 50 ohm
impedance, the transmitter load (and SWR) will be the same, anywhere along
the line, at any distance from the antenna.
The only time that line lengths will have any effect is when the impedance
at the antenna end is substantially different than the impedance of the
coax. If this weren't the case, we would ALL be using precisely cut pieces
of coax for each band.
73, Radio Randy N7CKJ
--- In FT897@yahoogroups.com, "D.J.J. Ring, Jr." <n1ea@...> wrote:
> The impedance of the antenna is transformed by the length of the coax,
> only being correct every 180Â° of electrical wave length, in addition,
> each coaxial connector introduces non-regular non-constant illenear
> impedances (except if type N). If you are talking about 144 MHz you
> are talking about considetable loss with RG8 mini coax, maybe 3 dB, so
> the measured SWR will often appear better than it really is.
> Check the coupler for problems, swap it out, join the coaxes with
> another double female, borrow spme coax, do the same thing.
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