Factory installed com antenna in the fin.

[shkdriver]

Hey Group,
I've got a 1975 Std, Cirrus new to me. I am replacing the com radio and in the process I deleted a poorly done splice in the coax cable with a rg-58 solder end.
After the bnc end was installed, I measured the resistance between the center and shield leads, I measured 0.9 ohms, thinking I had a stray shield wire or other problem, I removed the new bnc end and measured the bare end of the coax, still 0.9 ohms. Then I reinstalled the bnc end, same 0.9 ohms.
Does the special type of antenna in the tail has some weird ohm value?
Using a handheld I wouldn't miss if it fried, I measured swr and output watts, (using a MJF model 812B swr/watt meter)
watt output is 3-4, swr is 1.6-1.8 (in my garage, not ideal), Output watts are probably what the icom a-2 is putting out.

So, I'm not confident that the coax and antenna is safe for my radio, every other coax and antenna combination I ever measured was infinity between the ground shielding and the center lead.

I'm using a decent blue point digital multimeter.

Any Thoughts?

Thanks
Scott W

[Tim Newport-Peace[_2_]]

At 14:42 02 February 2016, shkdriver wrote:
>Hey Group,
>I've got a 1975 Std, Cirrus new to me. I am replacing the com radio and
in
>=
>the process I deleted a poorly done splice in the coax cable with a rg-58
>s=
>older end.
>After the bnc end was installed, I measured the resistance between the
>cent=
>er and shield leads, I measured 0.9 ohms, thinking I had a stray shield
>wir=
>e or other problem, I removed the new bnc end and measured the bare end
of
>=
>the coax, still 0.9 ohms. Then I reinstalled the bnc end, same 0.9 ohms.
> Does the special type of antenna in the tail has some weird ohm
value?
> Using a handheld I wouldn't miss if it fried, I measured swr and output
>wa=
>tts, (using a MJF model 812B swr/watt meter)
>watt output is 3-4, swr is 1.6-1.8 (in my garage, not ideal), Output
watts
>=
>are probably what the icom a-2 is putting out.
>
>So, I'm not confident that the coax and antenna is safe for my radio,
>every=
> other coax and antenna combination I ever measured was infinity between
>th=
>e ground shielding and the center lead.
>
>I'm using a decent blue point digital multimeter.
>
>Any Thoughts?
>
> Thanks
> Scott W
You said 'splice', did you really mean a taped join? Surely not!

Such joints are suspect at best and rarely moisture-proof. Moisture can
capillary up the Co-Ax and give very poor performance. A quality BNC or TNC
connector will give a moisture-proof seal, but any moisture already there
will be trapped.

Can’t say specifically about the Std. Cirrus, but in some fins there is
not sufficient height, and the aerial needs to be electrically longer. This
may give a surprising DC resistance.

Using a DC metre to judge an antenna is always risky.

The results from the SWR test don’t look too unreasonable, considering
there is a length of co-ax in the way. True SWR needs to be measured at the
antenna, but this is not always possible!

[Vaughn Simon[_2_]]

On 2/2/2016 9:42 AM, shkdriver wrote:
> Using a handheld I wouldn't miss if it fried, I measured swr and output watts,
>(using a MJF model 812B swr/watt meter)
> watt output is 3-4, swr is 1.6-1.8 (in my garage, not ideal), Output watts
are probably what the icom a-2 is putting out.
>
> So, I'm not confident that the coax and antenna is safe for my radio,
> every other coax and antenna combination I ever measured was infinity
> between the ground shielding and the center lead.
>
> I'm using a decent blue point digital multimeter.

Your SWR isn't very good, but it's not quite terrible either. Some
antennas are designed with a DC short, so (in itself) that .9 ohm
reading doesn't necessarily indicate any problem. Bottom line: Judging
from that SWR, your antenna is less than ideal. That said, if you get
good performance out of your system, I wouldn't worry too much about it.

[Heinz Gehlhaar]

On Tuesday, February 2, 2016 at 6:43:02 AM UTC-8, shkdriver wrote:
> Hey Group,
> I've got a 1975 Std, Cirrus new to me. I am replacing the com radio and in the process I deleted a poorly done splice in the coax cable with a rg-58 solder end.
> After the bnc end was installed, I measured the resistance between the center and shield leads, I measured 0.9 ohms,

<Snip snip>
Not too surprising. Look up Gamma matched antenna; the antenna is a dc short circuit.
Heinz

[[email protected]]

Most 'loaded' antennas will show little to no resistance. I just checked one and got .2 to .9 ohms. Recommend you get the BNC properly attached, then hook up your radio and give it a range check
JJ