[TowerTalk] Coax Seal yea or nay?

[K8RI]

On 5/19/2013 2:52 PM, Jim Lux wrote:
> On 5/19/13 11:03 AM, Larry Loen wrote:
>> Rescue Tape is recommended here in Arizona, but I have so far had trouble
>> getting it to seal as well.
>>
>> It self-seals very nicely.  The problem is sealing to (e.g.) the coax
>> itself or whatever other surface is of interest.  Also difficult IME
>> is the
>> very top of the connector.
>>
>> I still use coax seal (albeit sparingly) in some situations.  For
>> instance,
>> a small coating of it just outside of my junction box so that there's no
>> leak through the coax cable and grommet that goes into the box (in this
>> case, the cable does not enter from the bottom).
>>
>>
>
>
>
>
> So, I realize that we all have boxes of PL259s and SO-239 barrels to
> connect with, but why not just use a waterproof connector like N or DIN
> 7-16, and use no tape/seal/what-have-you?
>
> Just ballparking, looking at the wireman site, PL259s are about $4 each
> (for middle of the line).  N seems to be about $2 more.  DIN are pretty
> pricey ($20+ it seems)

The problem with "N" type connectors is they are only rated for 
something like 20# pull IIRC, but regardless they are very weak, 
mechanically.That's why I went to 3M flooded heat shrink which gives a 
connection that will support my weight.  I've even resorted to dumping 
some vinyl resin in before tightening the nut, Course that makes the 
connection permanent. The only wat to remove it is to cut it off.  If 
you do resort to the resin or epoxy, use something with low viscosity,
>
> I can see the concerns about the tiny center pin on an N at high powers,
> but it's not that much smaller than the old PL-259, and both are larger
> than the center conductor in 0.405" coax like RG-8/RG-213.

It's far more than enough to handle well over the legal limit in 
current. OTOH I've blown them out with a high SWR on 75 and 160 running 
QRO/legal limit.  I've also had nearby lightening strikes puncture the 
dielectric.

>
> 0.157" for PL-259 pin vs 0.120" for N center pin vs RG-8/RG-213 AWG13
> center conductor at 0.072".  The N has a stepped center pin: the
> skinnier center pin is more of a locator, the actual center conductor is
> 0.120" all the way through, and if properly mated, very little current
> will flow in the center pin (skin effect and all that).
>
> And, yes, installing an N is a big trickier than a UHF, especially if
> you want it to be waterproof.  And, it's easier to screw up the
> interface parts on the N, so it won't give you a nice match to 18 GHz or
> whatever, but I think even hosed up, it's probably better than the UHF.

When it comes to the average ability to do a good solder job on a UHF, 
I'd vote for the "N" type.  It may take a few to get it right, but after 
that they are easy.  A commercial stripper is well worth the money as it 
makes the right cuts at the proper lengths.


>
> And if you're not worried about impedance matching (which UHF connectors
> aren't), then a waterproof NEMA twist lock might actually be better bet.

Now I'd like to try that on 40 to see how the characteristics differ 
from the traces I have now.
>   The connector isn't all that long, so how much of a bump in VSWR are
> you really going to get?  (you would want to avoid a plug/receptacle
> configuration that is intermateable with any AC power at your location).
>
True, but it'd be worth it just to see the reactions when other hams see 
the connection the first time<:-))

Even UHF are hardly noticeable on the TDR through 2 meters.
Because I run SO2R with two towers, a typical run has around 10 
connectors.These all add up to well less than half a db, compared to the 
same length coax with only end connectors.

Because (when they are all hooked up)  I have well over a 100 connectors 
in the system


> (of course, actual tests I've seen on the web of UHF connectors on a VNA
> show not so wonderful performance at 400 MHz, but pretty decent down at
> 30 MHz (as in RL better than -20dB)

Well...if care is taken in making the connections, "N" type should be 
hardly noticeable.  My tri-bander is down at the moment so I can't run a 
RL on it, but I think I'll give the 2-meter circuit a check tonight (if 
my memory lasts that long)<:-))
On 40 and 20 at resonance RL is better than 60 db through a 200 ft run 
of LMR600.

BTW A VNA is a fantastic addition when using a tuner in the shack.
I use the AIM 4170C.  It's not all that expensive and after using a VNA 
I can't imagine operating without one.

73

Roger (K8RI)
>
> In any case, you've got non-coaxial paths as soon as you get to the
> physical antenna, and nobody is going to claim that the two wires coming
> out of any of a variety of baluns used at the feed of a yagi are some
> sort of constant impedance line (esp since the Yagi probably has a
> feedpoint impedance MUCH lower than any practical parallel line).  As
> with the PL-259, or using an AC plug, or binding posts, the length of
> the discontinuity is a very tiny fraction of a wavelength.
>
>
>
>
>
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