Topband: Beverage Coax & Matching Transformers (fwd)

[Tree N6TR]

Date: Mon, 22 Oct 2001 18:32:17 +1000
From: Sam Dellit <sam.dellit@bigpond.com>
To: Topband Reflector <topband@contesting.com>
Subject: Re: Topband: Beverage Coax & Matching Transformers
Resent-Date: Mon, 22 Oct 2001 13:17:03 -0400
Resent-From: topband-admin@contesting.com
Resent-To: topband-owner@contesting.com

g'day tom & the topband gang

my beverage farm continues to grow but still needs a little fine tuning

i am running 6 beverages of between 150 metres and 400 metres N - E - S
all about 5 metres high to clear the horses

all are separately fed with 100 metres of RG11 and 75ohm to 500ohm matching
transformers

all seems to go ok but some comments sought :

(1) for reasons of cost, i am using a very cheap "RG11" of chinese
manufacture (price is 80Acents/metre,about 12UScents a foot), it has double
shield of aluminium foil and ALUMINIUM braid, not copper; also the centre
conductor is copper clad, single strand STEEL. i went for RG11 rather than
RG6 because the runs are quite long and assumed the RG11 would be less
lossy, but measured DC resistances on this cheap RG11 are (for a 100 metre
run), 40 ohms for centre conductor and 30 ohms for the outer shield - seems
very high and if i eventually go to 200metre feeds it will be even worse.
don't really want to have to install preamps at the beverages, we just have
too many lightning storms in south east queensland and remote lightning
protection is a real problem. would i get less loss with a better quality
RG6 cable compared to the cheap RG11? have you measured actual attenuation
values on typical RG6 and RG11 cables in long runs? the CATV suppliers
aren't much interested in perfomance at 2MHz - only 2GHz.

(2) my matching transformers are designed for 75ohms to 500ohms and is
simply the autotransformer configuration - 8 turns of 26 B&S close wound and
tapped at 3 turns on an Amidon BN-72-202. ferrite balun former. why do you
suggest separate primary and secondary windings? and why do you suggest
separate earths? while i have not yet done so, i intend to slide a bundle of
lossy toroids or equivalent over each coax at 50 metre intervals - won't
that be enough to dispose of the common mode signals?

TIA

73s gd dx de vk4zss sam dellit

----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
To: <Topband@contesting.com>; "Robert Wood WW5AJ"" <w5aj@arrl.net>
Sent: Sunday, October 21, 2001 8:51 PM
Subject: Re: Topband: Beverage coax


> > There appears to be several flavors of RG-6, the heaviest and most
> > cost is one claiming extra shielding. (quad shielding?)   The price
> > looks ok even for the extra shielding at 60$/500 ft.
> > My understanding is better shielding is less likely to effect the
> > beverage pattern.
>
> Hi Bob,
>
> The shield is the least of things affecting coax radiation problems
> on HF through upper VHF. Thick multiple-shields can be a good
> thing because they can take a lightning hit with less worry about
> opening up the cable shield, but forget about signal ingress
> problems from poor shielding unless the cable is incredibly poor.
> The general rule of thumb is if you strip the cable back and can't
> see the cable's internal dielectric looking through the shield, it is a
> good enough shield!
>
> Most concerns in cable selection are related to mechanical and life
> worries. Flooded cables should be used for all underground wiring,
> wiring that lays in wet areas, or wiring that rodents and animals
> might chew. Other than than problem and loss concerns, one cable
> is about as good as another.
>
> The biggest problem by far with interaction and signal ingress are
> common mode currents, and that depends on connections at the
> ends of the cable and how the feedpoint is arranged.
>
> Do not route the cable  parallel to the antenna and close to the
> antenna. Use an isolation-type matching transformer at the
> antenna. Don't worry about Faraday shields and other things in the
> transformer, they are for the most part useless. Just use a straight
> primary/secondary-type transformer using the correct core material
> (73 material works about best in this application).
>
> You can ground the cable to a separate ground rod several feet
> from the antenna (I usually don't bother, since my cables are all
> buried), but never ground it to the same ground as the Beverage
> feedpoint.
>
> 73, Tom W8JI
> W8JI@contesting.com
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