Hi Mauri,
You know, next time I'm in Italy (this summer), you and I ought to sit down
with some wine and cheese and have a coax dual <grin>!
Like you and others, I've successfully used RG8X for years on 80 and 40 with
moderately high power.
Other the other hand, perhaps I bought some cheap RG8X at some point,
because it certainly did develop shorts. I found myself cutting the line in
half, then in half again, etc. till I found them. I've always presumed it
was voltage breakdown, not heat, since I've always kept the SWR low, but who
knows, I might have loaded it up on an odd band at some point. But on high
power, I think not. Possibly just a bad batch of coax. Interestingly, it
did not fail at points where it was bent, but rather right in the middle of
a straight stretch.
It's like everything else we do as hams..make do with what we've got. If
what we have is RG8X, we use it, myself included. But, I find myself
considering alternatives when starting from scratch with new antenna
projects.
Great discussion, though. Forced me to dive back into the Handbook and
other manuals. Thanks!
73,
Gary W2CS
| -----Original Message-----
| From: owner-towertalk@contesting.com
| [mailto:owner-towertalk@contesting.com]On Behalf Of i4jmy@iol.it
| Sent: Friday, March 02, 2001 9:24 AM
| To: w2cs@ipass.net
| Cc: aa4lr@arrl.net; w2cs@ipass.net; towertalk@contesting.com
| Subject: RE: [TowerTalk] Coax Rating
|
|
| Hi Gary,
|
| I didn't guess, as well as I don't take "as is" a claimed number.
| I prudentially estimated a reasonable breakdown voltage taking into
| account cable dimensions, dielectric thickness and the dielectric
| constant of the used insulating material.
| I don't know where does the 300V RMS figure cames from, but if the
| sizes I estimated for an RG8X cable are comparable with the real ones,
| that breakdown voltage of 300V is by far not a realistic figure.
| Considering the distance between the concentrical conductors, even with
| an air gap (dielectric constant 1), the breakdown voltage is higher
| than 300V.
| I went to that http://www.thewireman.com/coaxdata.pdf but I don't think
| that is a page from the Bible.
| For example an RG142 (a teflon insulated coax cable) is reported with
| the same 1400V voltage of an RG58 that uses a polyethylene dielectric,
| another strange data is the mini RG174 with 1100V, at least if compared
| with the RG58 data shown.
|
|
| 73,
| Mauri I4JMY
|
|
| > ---------- Initial message -----------
| >
| > From : "Gary J. Ferdinand W2CS" <w2cs@ipass.net>
| > To : <i4jmy@iol.it>, <aa4lr@arrl.net>, <w2cs@ipass.net>,
| <towertalk@contesting.com>
| > Cc :
| > Date : Fri, 2 Mar 2001 07:26:23 -0500
| > Subject : RE: [TowerTalk] Coax Rating
| >
| > Mauri,
| >
| > Comments interspersed with yours.
| >
| >
| > | This stuff with RG8X is getting interesting.
| > | Well, after some researches it seems to me that what is sold in US
| as
| > | an RG8X has a size that's not so different from an RG59, formerly
| > | 0.242".
| >
| > That is true. The physical, outer diameter is extremely close. It
| uses the
| > same "sleeve" adapter to PL259 coax connectors as RG59.
| >
| > | The voltage rating of an RG59 with a solid polyethylene is 2.3 KV
| RMS.
| > | But the inner diameter of an RG8X must be a bit bigger since the
| cable
| > | is 50 OHms instead of 75, and the dielectric constant of foam PE is
| > | less than with a compact type.
| >
| > Solid poly dielectric has a much higher voltage rating than foam.
| Material,
| > not size per se, is the dominant factor. My data shows RG59 at more
| like
| > 1.7KV rms.
| >
| > | The lower dielectric constant is 1.5 for foam polyethylene and 2.26
| for
| > | solid polyethylene and all this leads to a consequential decrease in
| > | the voltage rating of the cable.
| > | Considering a safe margin, the voltage rating for a cable of that
| size
| > | and with foam poyethylene dielectric is minimum the half of what it
| is
| > | with a compact poyethylene.
| >
| > Why guess? Check out information available on the web. The best
| info I've
| > been able to get states that RG8X has a rating of 300Vrms. This is
| from
| > http://www.thewireman.com/coaxdata.pdf . It is interesting to note
| that
| > RG174 is 1100V, RG58 is 1400V, RG59 is 1700V and RG8 is 4000V.
| >
| > | Neglecting ohmic and expecially dielectric losses (low because foam
| > | type and because frequency is not high), and further halving the max
| > | applicable voltage (1.150/2=575V) to be "really" reliable (halving
| the
| > | voltage makes the power decreased to 1/4), with an SWR of 1:1 a safe
| > | max power for that cable could be (575x575)/50=6612 Watt.
| >
| > Interesting approach. Now let's try it will real numbers:
| >
| > Let's assume what you did that the load is purely resistive and is
| exactly
| > 50 ohms, as is the source. So, VSWR is 1:1. In that case we can use
| DC
| > calculations:
| >
| > P=E*E/R -> E=SQRT(PR). At 1500W output E=SQRT(1500*50)=SQRT(75000)
| =274
| > volts (rms).
| >
| > So, unless I've slipped a decimal point it would appear that in the
| best
| > case of an ideal match, no sharp bends in the coax, undamaged,
| lossless
| > cable, and with 1500W output, we come out 26 volts short of the rated
| > breakdown voltage of the cable, or about 9% short of breakdown.
| >
| > VSWR by definition is the ratio of maximum to minimum voltage on the
| > transmission line. With a VSWR of 1.3:1 the maximum value is 30%
| higher
| > than the minimum. Remember, this thread started with the assumption
| he was
| > feeding a 3/2-wave antenna. Do not assume this is a perfect 50-ohm
| load.
| > (More like 70+, or a 1.5:1 VSWR.)
| >
| > Now throw in typical ham practicalities... The bend where the coax
| goes
| > into the house, a wrap of the coax around the center insulator, the
| coax
| > whipping in a strong wind. Coax on the ground mistakenly having been
| walked
| > on. All these things potentially, though in small ways to be sure,
| change
| > the breakdown rating by altering the physical characteristics of the
| foam
| > dielectric.
| >
| > Finally, throw in the 1 db or so loss typically found in such coax at
| 7MHz
| > over a 100 ft run a year or so after the coax has been in the
| elements, and
| > you have the recipe for a transmission line failure. Other options
| are more
| > prudent.
| >
| > I'd divide power by two and even then treat this coax very carefully!
| >
| > 73,
| >
| > Gary W2CS
| >
| >
| >
| >
| >
|
|
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