OTA Cabling: RG-6, RG-59, RG-11, Twin-Lead, Crimping, Other Tips - Page 11 - Canadian TV, Computing and Home Theatre Forums

roger1818 · 2009-07-21, 03:31 PM

Quote:

Originally Posted by balm

I managed to find some foam insulated twinlead by Nexxtech - the package says "super low-loss" - 20 gauge.

All twinlead is insulated. What you need is shielded twinlead.

balm · 2009-07-21, 03:39 PM

Quote:

All twinlead is insulated. What you need is shielded twinlead.

Just using the packaging wording, but yes, I understand, and its NOT shielded

So just to be clear on the types for 300ohm twin lead (20 ga):

1- standard, insulated
2- insulated, foam core
3- insulated, foam core, shielded

Now should I worry about passing the foam core, UNSHIELDED, twin lead thru the bow-tie reflector (metal) and on into the standoff and feed point. Only very short pieces (1-2 ft) will be used to go from antenna to amp, or antenna to coax line...

balm · 2009-07-22, 01:17 PM

Would it make any sense to add braided shielding jacket around the standard unshielded twin lead, or is this worse, meaning it will cause more interference then it would reject...???

stampeder · 2009-07-22, 01:21 PM

Yes, it would turn it into shielded twin-lead and you would get rid of the need for standoff insulators and line twists. Do you have an easy method of doing that?

twinz · 2009-07-22, 02:41 PM

In searching for some shielded twin lead to experiment with in making a coax balun for a stacked pair of 4221hd's, I saw a couple of sites and forums from some ham operators that are making their own using 2 lengths of coax taped together with the shielding soldered together and grounded for getting into and out of their shacks without interference from storm window frames and aluminium siding (I can't remember the variety of coax though). Are they on to something or is there something fundamentally wrong with the idea?

balm · 2009-07-22, 04:31 PM

Quote:

Yes, it would turn it into shielded twin-lead and you would get rid of the need for standoff insulators and line twists. Do you have an easy method of doing that?

Stampeder, today I purchased some shielding, just shielding alone, in a sock-like form, to slip over the twin lead. There are different sizes, but most places have no more of the correct size for the job (and wont be getting any more either!). I got some 3/4" shield, and thinking of simply sliping the twin lead into it, folding over the excess width, and fastening snugly (but not too tight) to the twin lead using some tie-wraps...not sure thats the best way, but for short lengths so what if I use several tie-wraps

Ill post some pics...

balm · 2009-07-22, 04:49 PM

Et voila!

http://s638.photobucket.com/albums/u...t=IMG_0296.jpg

JamesK · 2009-07-23, 11:32 AM

Doing so would also destroy the transmission line characteristics of that cable. Placing a shield on would significantly affect the distributed capacitance between the conductors, which will in turn affect the impedance and loss. Shielded transmission lines have to be engineered as such.

balm · 2009-07-23, 11:46 AM

I was afraid of something like that, see my post #156...

Oh well I guess its back to using standoffs, what a PITA!

stampeder · 2009-07-23, 11:59 AM

Quote:

Originally Posted by JamesK

Shielded transmission lines have to be engineered as such.

When Balm first posted the idea of putting shielding on twin-lead I had a feeling there was a problem with it but I couldn't put my finger on it. Then after your post I remembered the old trick of "tuning" twin-lead by sliding a loop of aluminum foil along it, which was an actual TV repair tip found in many old manuals for getting rid of certain types of interference. It is the exact same principle that Jerrold tried with this old antenna design:

http://www.digitalhome.ca/forum/show...02&postcount=2

And its the same principle used back in the mid 1980s to pirate encrypted cable TV channels.

Thanks for the reminder that DIY cable shielding is a black art, JamesK.

balm · 2009-07-23, 03:01 PM

Quote:

Thanks for the reminder that DIY cable shielding is a black art, JamesK.

Just to confirm - I shouldnt try adding the sheilding...just use standoffs...

balm · 2009-07-23, 11:59 PM

I am trying to recalculate the loop length for the coax balun, and need to know the velocity factor which differs for RG-59 cable (with standard dielectric) versus RG-59 (with gas-injected foam dielectric)....

other than markings, how does one tell the difference based on a visual inspection of the insulator..?

holl_ands · 2009-07-24, 01:30 AM

Foam dielectric in "RG-59" is like what is found in typical RG-6:
http://www.busytrade.com/common/image.php?file=1268976
It feels textured and slightly rough....FOAM with very tight cell structure.
It's OPAQUE and very EASY to deform when you squeeze it....

Standard RG-59 polyethylene (PE) dielectric is quite a bit harder:
http://en.wikipedia.org/wiki/File:RG-59.jpg
It looks and feels very smooth....because it's PLASTIC....and slick like teflon....
It's TRANSPARENT and very DIFFICULT to deform when you squeeze it....

BTW: Mil-Specs for RG-59 require single 94 or 97% copper braid shield
with 66% "nominal" velocity factor....and does not allow foam dielectric....

Nonetheless numerous manufacturers offer "RG-59" with foam dielectric,
different velocity factors and even multiple shields....go figure, seems to
me they should be labeled as some variation of RG-6....or something else....

=============================================
If you have access to an RF signal source and a signal level meter,
see Figure 12 to determine velocity factor:
http://www.qsl.net/vk5br/TransLines.htm
Measurement frequency doesn't necessarily have to be in VHF/UHF TV band....

balm · 2009-07-24, 02:41 AM

holl_ands,

Thanks for that informative post but I noticed the designation RG-59 versus RG-59/U (Universal). Do you know the significance of these different designations?

holl_ands · 2009-07-24, 11:42 AM

Originally, the "/U" (for Universal use, vice Airborne, Shipboard, etc.) was a
part of the formal cable nomenclature, which people usually dropped for
informal use. Since the slash designator was already defined for use with
ANY piece of military equipment, slash variations were never defined for
unique CABLE applications, such as buried, armored or low-smoke
(PVC smoke is highly toxic). Whatever /X found in manufacturer
advertisements is strictly a product of their active imaginations.

The whole ugly story:

Formal MIL-SPEC part numbers for RG-59 are as follows (each spec revision is superceeded):

1. "RG-59A/U" per MIL-C-17/29A (old original, 1950's???)

2. "RG-59B/U" per MIL-C-17/29B (28 February 1964)

3. "M17/29-RG59" per MIL-C-17/29C (24 April 1978):
http://assist.daps.dla.mil/quicksear...ent_number=956

4. "M17/184-00001" per MIL-C-17/184B (20 February 1991):
http://assist.daps.dla.mil/quicksear...nt_number=1065

ALL of these describe the SAME cable construction with a solid polyethylene dielectric,
single copper coated steel braid with either 94.4% or 96.7% coverage and 65.9%
velocity factor. The only notable change over time was allowing 22 pf vice 20 pf
max capacitance per foot.

Also note first two "formal" part numbers includes the MIL-SPEC revision letter,
which is usually dropped for convenience (esp since they are all the SAME).

Note that the term "RG-59" doesn't even appear in the latest MIL-SPEC!!!!
Which helps to explain why some cable manu's currently sell whatever they want, with
whatever velocity factor, one or multiple shields and attenuation they want as "RG-59"....

2009-07-22, 01:17 PM	#153
balm Veteran Join Date: Jan 2009 Location: Montreal Posts: 2,248	shielding twin lead Would it make any sense to add braided shielding jacket around the standard unshielded twin lead, or is this worse, meaning it will cause more interference then it would reject...???

2009-07-23, 11:59 PM	#162
balm Veteran Join Date: Jan 2009 Location: Montreal Posts: 2,248	Cable Info I am trying to recalculate the loop length for the coax balun, and need to know the velocity factor which differs for RG-59 cable (with standard dielectric) versus RG-59 (with gas-injected foam dielectric).... other than markings, how does one tell the difference based on a visual inspection of the insulator..?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

2009-07-22, 01:21 PM	#154
stampeder OTA Forum Moderator Join Date: Jan 2005 Location: North Delta, BC (96Av x 116St) Posts: 23,338	Yes, it would turn it into shielded twin-lead and you would get rid of the need for standoff insulators and line twists. Do you have an easy method of doing that?

2009-07-22, 02:41 PM	#155
twinz Rookie Join Date: May 2009 Location: Ajax Posts: 28	In searching for some shielded twin lead to experiment with in making a coax balun for a stacked pair of 4221hd's, I saw a couple of sites and forums from some ham operators that are making their own using 2 lengths of coax taped together with the shielding soldered together and grounded for getting into and out of their shacks without interference from storm window frames and aluminium siding (I can't remember the variety of coax though). Are they on to something or is there something fundamentally wrong with the idea?

2009-07-22, 04:49 PM	#157
balm Veteran Join Date: Jan 2009 Location: Montreal Posts: 2,248	Et voila! http://s638.photobucket.com/albums/u...t=IMG_0296.jpg

2009-07-23, 11:32 AM	#158
JamesK Veteran Join Date: May 2009 Location: Mississauga Posts: 5,039	Doing so would also destroy the transmission line characteristics of that cable. Placing a shield on would significantly affect the distributed capacitance between the conductors, which will in turn affect the impedance and loss. Shielded transmission lines have to be engineered as such.

2009-07-23, 11:46 AM	#159
balm Veteran Join Date: Jan 2009 Location: Montreal Posts: 2,248	I was afraid of something like that, see my post #156... Oh well I guess its back to using standoffs, what a PITA!

2009-07-24, 01:30 AM	#163
holl_ands Veteran Join Date: Nov 2008 Location: San Diego, CA Posts: 1,592	Foam dielectric in "RG-59" is like what is found in typical RG-6: http://www.busytrade.com/common/image.php?file=1268976 It feels textured and slightly rough....FOAM with very tight cell structure. It's OPAQUE and very EASY to deform when you squeeze it.... Standard RG-59 polyethylene (PE) dielectric is quite a bit harder: http://en.wikipedia.org/wiki/File:RG-59.jpg It looks and feels very smooth....because it's PLASTIC....and slick like teflon.... It's TRANSPARENT and very DIFFICULT to deform when you squeeze it.... BTW: Mil-Specs for RG-59 require single 94 or 97% copper braid shield with 66% "nominal" velocity factor....and does not allow foam dielectric.... Nonetheless numerous manufacturers offer "RG-59" with foam dielectric, different velocity factors and even multiple shields....go figure, seems to me they should be labeled as some variation of RG-6....or something else.... ============================================= If you have access to an RF signal source and a signal level meter, see Figure 12 to determine velocity factor: http://www.qsl.net/vk5br/TransLines.htm Measurement frequency doesn't necessarily have to be in VHF/UHF TV band....

2009-07-24, 02:41 AM	#164
balm Veteran Join Date: Jan 2009 Location: Montreal Posts: 2,248	holl_ands, Thanks for that informative post but I noticed the designation RG-59 versus RG-59/U (Universal). Do you know the significance of these different designations?

2009-07-24, 11:42 AM	#165
holl_ands Veteran Join Date: Nov 2008 Location: San Diego, CA Posts: 1,592	Originally, the "/U" (for Universal use, vice Airborne, Shipboard, etc.) was a part of the formal cable nomenclature, which people usually dropped for informal use. Since the slash designator was already defined for use with ANY piece of military equipment, slash variations were never defined for unique CABLE applications, such as buried, armored or low-smoke (PVC smoke is highly toxic). Whatever /X found in manufacturer advertisements is strictly a product of their active imaginations. The whole ugly story: Formal MIL-SPEC part numbers for RG-59 are as follows (each spec revision is superceeded): 1. "RG-59A/U" per MIL-C-17/29A (old original, 1950's???) 2. "RG-59B/U" per MIL-C-17/29B (28 February 1964) 3. "M17/29-RG59" per MIL-C-17/29C (24 April 1978): http://assist.daps.dla.mil/quicksear...ent_number=956 4. "M17/184-00001" per MIL-C-17/184B (20 February 1991): http://assist.daps.dla.mil/quicksear...nt_number=1065 ALL of these describe the SAME cable construction with a solid polyethylene dielectric, single copper coated steel braid with either 94.4% or 96.7% coverage and 65.9% velocity factor. The only notable change over time was allowing 22 pf vice 20 pf max capacitance per foot. Also note first two "formal" part numbers includes the MIL-SPEC revision letter, which is usually dropped for convenience (esp since they are all the SAME). Note that the term "RG-59" doesn't even appear in the latest MIL-SPEC!!!! Which helps to explain why some cable manu's currently sell whatever they want, with whatever velocity factor, one or multiple shields and attenuation they want as "RG-59"....