[600MRG] Coax shielding effectiveness

Wed Dec 11 09:11:27 EST 2019

I did get some FSJ1-50 jumpers from Andrew, F1B-PNMBM-1M, and they are way
better than the Pasternak "100% shield" cables, but they still leak at the
ends.
In the middle, essentially no leak.  So I suspect the ends aren't being
done completely correctly.  I haven't carved into one yet to find out.

That makes me think about making my own cables, which makes me think in the
direction of "Superscreen" or "Double Superscreen" cables.
Unfortunately I have not been able to look up any source for those cables,
much less the connectors.

Heavy sigh..  Designing sensitive receivers isn't that big a problem, but
it seems all the ancillary bits are determined to drive me nuts.

On Mon, Dec 9, 2019 at 5:34 PM Gedas <w8bya at mchsi.com> wrote:

> Labas Ed !
>
> Wow those are some awesome relays to have that kind of isolation, esp at
> 1296. I am WAY behind on my antenna work here as well. Just today I had to
> lower the tower AGAIN after I found yet more lightning damage from last
> year. Once done I may be able to put up the next experimental 630m antenna
> I am pondering. I am needing a VLF QSO fix.
>
> Speaking of which, Ed made a special stop near my QTH last year so we
> could have a very memorable eye-ball QSO. I will have to find that picture
> and post it to the group. Merry X-Mas if we do not chat before that time
> Ed. 73
>
> Gedas, W8BYA
>
> Gallery at http://w8bya.com
> Light travels faster than sound....
> This is why some people appear bright until you hear them speak.
>
> On 12/9/2019 6:35 PM, Edward R Cole wrote:
>
> Pretty much how I measure RF isolation.  In commercial communications:
> typically important to avoid desense in duplex operations.  Use of
> dual-shielded coax to obtain 90-dB.
>
> In ham radio its mostly port isolation in TR relays.  I did essentially
> the same process as Gedas.  My eme preamps are the most sensitive so look
> at the leakage level with 10-100w in the Tx port.  I could not see any
> signal at 100-dB on 1296 with my Narda relays (cited at 80-dB).  Since my
> max transmission power is 600w (+58 dBm) that keeps leakage into my preamps
> way below 0 dBm.
>
> I run Heliax transmission line and see no leakage signal into the Preamp
> on 144-MHz.  Space loss from antenna to receiver is less so it sees the
> transmitted signal if radiated.  So definitely need terminations in these
> tests.
>
> Thanks for the info, Gedas.
>
> My 630m inverted-L will not be in-service until next spring as I did not
> get my radial system planted this year (new lawn).  Also base loading coil
> is to be installed inside new fiberglass wx-proof cabinet which will also
> house my 2m-1500w PA & 50v PS.
>
> Winter arrived!
>
> 73, Ed - KL7UW
>
> At 10:14 AM 12/9/2019, you wrote:
>
> Content-Type: multipart/alternative;
>  boundary="------------78ED9BB5007D8DB12F1545D4"
> Content-Language: en-US
>
> Hi Dave. Funny you should ask. I am right in the middle of making a video
> where I test about 10 different (and common) coaxial cables specifically
> for their SE. The project is on and off as I just have too many pokers in
> the fire right now.
>
> It is not as simple as one may think to measure the SE accurately esp if
> the cable is of high quality and has a high SE. If I were still employed
> and had access to my old lab and shielded enclosures and able to generate &
> measure known RF fields from DC to daylight it would be a much, much easier
> task. Having to do it in my basement lab makes it a much more challenging
> project. In the end I chose to arrive at the SE by measuring the cross-talk
> between two identical coax cables.
>
> My methodology is to lay two identical cables of equal length next to each
> other on a plywood table. Both far ends of the coax cables are terminated
> in 50 ohm loads. One cable is driven with a tracking generator (then
> broadband power amplifier) while the other coax fed into the input of my
> spectrum analyzer. The amount of cross-talk between the two cables can then
> be used to directly see what the SE was. It gets hairy if the cables have a
> high SE as the SA has a limited sensitivity (NF) and I can only generate
> several watts of broadband RF from DC to about 1 GHz.
>
> Until that video is done you will have to go by OEM data. FSJ1-50 will
> have an extremely high SE (well over 100 dB). If you need some help looking
> up the exact OEM data just hollar. 73
>
> Gedas, W8BYA
>
> Gallery at http://w8bya.com
> Light travels faster than sound....
> This is why some people appear bright until you hear them
> speak.
>
> On 12/9/2019 1:37 PM, david vanhorn wrote:
>
> I've been searching around without much success.
> I'm trying to find data sheets or good measurements of shielding
> effectiveness of the various coax types, including FSJ1-50 for this band.Â
> I'm not seeing anything talking much below 100MHz.
>
> Can someone point me in the right direction?
>
> --
> K1FZY (WA4TPW) SKÂ  9/29/37-4/13/15
>
>
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> 73, Ed - KL7UW
>   http://www.kl7uw.com
> Dubus-NA Business mail:
>   dubususa at gmail.com
>
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-- 
K1FZY (WA4TPW) SK  9/29/37-4/13/15
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