[Antennas] Only dumb question is the one not asked
Chris BONDE
[email protected]
2002年3月29日 17:26:58 -0800
I like this answer. I hope that it is within the theory.
When I first learned about the radiating coax, I thought why not? So I
took the coax run off the antenna at the prescribed 90degrees, but at the
same horizontal as the legs of the droopy dipole. So then I had the main
part of the dipole radiating plus the outside of the coax. Hey, and
hay, I had good results. Yes, I only had 100w or so out, so I tried to
make use of everthing.
Chris opr VE7HCB
At 06:41 PM 2002年03月29日 -0600, [email protected] wrote:
>RF current flowing on the outside of a coaxial feedline is
>not a normal situation. It is caused by an abnormal
>condition. You get that by connecting a balanced antenna
>(dipole) to an unbalanced feed line (coax). Yes, you can
>reduce and even eliminate the unwanted current (and
>radiation) by using a choke at the antenna connection.
>However, a better solution is to install a balun at the
>antenna and connect the feedline to the balun. This
>eliminates the cause of the common mode current and results
>in a non-radiating feedline as well as a more efficient
>transfer of power to the antenna.
>>Is this realy needed? Well, if you are concerned about
>common mode currents running down the coax and causing
>radiation and hot spots in the shack, yes. It is
>important.
>>I have found that it really only becomes important if your
>power levels are over 300 watts (output) or in situations
>where every fraction of a percentage point in efficiency
>makes a difference.
>>It does not matter much at power levels around 100 watts or
>so. At that level, I have not seen much difference in
>connecting the coax directly, using the decoupling choke
>trick or going with a balun.
>>Regards,
>Frank Kamp
>K5DKZ
>>>Harvey&Bessie wrote:
> >
> > The impedance right at the feed point is (when SWR is low) close to 50
> > ohms. the coiled co-ax near the feed point forms an RF-choke only for
> > the outer conductor (the field of the inner conductor is entirely
> > contained within the outer conductor acting as a shield) so, the RF
> > "seeing" a higher impedance (several hundred ohms) in the path down the
> > outer conductor, as compared to the 50-ohm impedance of the inner
> > conductor, does not travel down the outside of the outer conductor.
> > As for connecting a "ground" directly to the outer conductor at the feed
> > point -- how would you propose doing that? The antenna, hopefully, is
> > elevated some distance from earth -- the source of any true ground
> > connection -- any wire lead you connect there has to have considerable
> > length to get to "ground." It is still ground for DC, but not for RF. In
> > fact if it is exactly (or nearly) one-quarter wavelength long it is no
> > ground at all but an open circuit at RF!
> > Harvey/W4TG
> >
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