Hello Group,
I am currently restoring a Hygain 18AVt vertical antenna for our local AR
club.

I have dis-assembled the bracket on which the feedpoint is mounted. I have
discovered that the centre conductor of the SO239 to the first vertical
member is fed with a short length of braided copper.

My question is, should I use a piece of braid or stranded copper wire, or a
piece of solid copper wire in a 1/2 turn coil to minimise any strain on the
joints?

I was led to believe that braid isn't a good earthing medium and wonder if
the same applies for feeding RF.

The maximum transmitting power won't be over 200 watts.

I would appreciate some input here.

73
John
VK2KCE

VK2KCE wrote:
Hello Group,
I am currently restoring a Hygain 18AVt vertical antenna for our local AR
club.

I have dis-assembled the bracket on which the feedpoint is mounted. I have
discovered that the centre conductor of the SO239 to the first vertical
member is fed with a short length of braided copper.

My question is, should I use a piece of braid or stranded copper wire, or a
piece of solid copper wire in a 1/2 turn coil to minimise any strain on the
joints?

I was led to believe that braid isn't a good earthing medium and wonder if
the same applies for feeding RF.

The maximum transmitting power won't be over 200 watts.

I would appreciate some input here.

From an electrical standpoint, any of those are equally good, with the
possible exception of the half turn option. Braid has more RF resistance
than an equivalent flat or round conductor, but it'll be negligible in
this application. Putting a half turn in the wire will add some
reactance, which might possibly be noticeable on 10 meters depending on
the wire length and size of the half turn. If it is, it'll affect
resonance but won't have an appreciable effect on efficiency.

Roy Lewallen, W7EL

Stefan Wolfe wrote:

Actually braid has muxh less 'resistance' than a flat or round conducor at
RF due to greatly increased surface area and skin effect.

Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those frequencies
that Litz wire can provide some advantage. But it's made of separately
insulated strands. Because of skin effect, the current at HF (or
wherever the wire diameter is at least several skin depths) is only on
the outside surface of the braid, not the outside surfaces of all the
wires. The extra loss comes from the necessity of the current moving
from one set of wires to another as the original set goes under an
adjacent group. Surface roughness in itself can significantly increase
RF resistance (cf. Johnson and Graham, _Signal Propagation - Advanced
Black Magic_, Sec. 2.11), but the braid structure increases the
resistance more yet.

I know Tom, W8JI, has measured the impedance of solid strap and compared
it to braided shield, and confirmed that the shield has substantially
higher RF resistance. I don't see measurement results on his web site,
but he briefly discusses the phenomenon at
http://www.w8ji.com/skindepth.htm.

One other note - for the same surface area, a round conductor has the
least RF resistance of any conductor shape. The reason is that the
current is evenly distributed on the surface. On other shapes it's not,
resulting in higher resistance per unit length.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Stefan Wolfe wrote:
Actually braid has muxh less 'resistance' than a flat or round
conducor at RF due to greatly increased surface area and skin effect.

Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those
frequencies that Litz wire can provide some advantage. But it's made of
separately insulated strands. Because of skin effect, the current at HF
(or wherever the wire diameter is at least several skin depths) is only
on the outside surface of the braid, not the outside surfaces of all
the wires. The extra loss comes from the necessity of the current
moving from one set of wires to another as the original set goes under
an adjacent group. Surface roughness in itself can significantly
increase RF resistance (cf. Johnson and Graham, _Signal Propagation -
Advanced Black Magic_, Sec. 2.11), but the braid structure increases
the resistance more yet.

I know Tom, W8JI, has measured the impedance of solid strap and
compared it to braided shield, and confirmed that the shield has
substantially higher RF resistance. I don't see measurement results on
his web site, but he briefly discusses the phenomenon at
http://www.w8ji.com/skindepth.htm.

The higher RF resistance of braid is very noticeable in tube power
amplifiers at HF, where the circulating currents are magnified by the Q
of the tank circuit. If the connections between the coil taps and the
bandswitch contacts are made from braid, they can run very hot, while
thin strips of solid copper give no problems at all.

For antennas, the main concern about using braid is corrosion, which
will insulate the strands from one another and greatly increase the RF
resistance. This is why the loss of normal braid-covered coax increases
dramatically if water gets under the jacket.

However, the original question was about a very short braid connection
in a non-critical application. There must be hundreds of 18AVTs out
there, all with corroded braid. Nobody notices the difference, so that
proves it's non-critical, right? :-)

The purpose of this braid is to prevent failures of the solder joint to
the SO-239 after a few months/years of flexing in the wind. John's
original suggestion of a small loop of insulated stranded wire would do
equally well - one turn around a pencil, say. Sealing the two solder
joints with hot-melt glue will finish the job nicely.

The tiny additional series inductance will not be significant at HF.
When the elements are adjusted to length, it will vanish completely.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On 3 Nov, 01:27, Ian White GM3SEK wrote:
Roy Lewallen wrote:
Stefan Wolfe wrote:
Actually braid has muxh less 'resistance' than a flat or round
conducor at RF due to greatly increased surface area and skin effect.

Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those
frequencies that Litz wire can provide some advantage. But it's made of
separately insulated strands. Because of skin effect, the current at HF
(or wherever the wire diameter is at least several skin depths) is only
on the outside surface of the braid, not the outside surfaces of all
the wires. The extra loss comes from the necessity of the current
moving from one set of wires to another as the original set goes under
an adjacent group. Surface roughness in itself can significantly
increase RF resistance (cf. Johnson and Graham, _Signal Propagation -
Advanced Black Magic_, Sec. 2.11), but the braid structure increases
the resistance more yet.

I know Tom, W8JI, has measured the impedance of solid strap and
compared it to braided shield, and confirmed that the shield has
substantially higher RF resistance. I don't see measurement results on
his web site, but he briefly discusses the phenomenon at
http://www.w8ji.com/skindepth.htm.

The higher RF resistance of braid is very noticeable in tube power
amplifiers at HF, where the circulating currents are magnified by the Q
of the tank circuit. If the connections between the coil taps and the
bandswitch contacts are made from braid, they can run very hot, while
thin strips of solid copper give no problems at all.

For antennas, the main concern about using braid is corrosion, which
will insulate the strands from one another and greatly increase the RF
resistance. This is why the loss of normal braid-covered coax increases
dramatically if water gets under the jacket.

However, the original question was about a very short braid connection
in a non-critical application. There must be hundreds of 18AVTs out
there, all with corroded braid. Nobody notices the difference, so that
proves it's non-critical, right? :-)

The purpose of this braid is to prevent failures of the solder joint to
the SO-239 after a few months/years of flexing in the wind. John's
original suggestion of a small loop of insulated stranded wire would do
equally well - one turn around a pencil, say. Sealing the two solder
joints with hot-melt glue will finish the job nicely.

The tiny additional series inductance will not be significant at HF.
When the elements are adjusted to length, it will vanish completely.

--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek- Hide quoted text -

- Show quoted text -

On 3 Nov, 01:27, Ian White GM3SEK wrote:
Roy Lewallen wrote:
Stefan Wolfe wrote:
Actually braid has muxh less 'resistance' than a flat or round
conducor at RF due to greatly increased surface area and skin effect.

Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those
frequencies that Litz wire can provide some advantage. But it's made of
separately insulated strands. Because of skin effect, the current at HF
(or wherever the wire diameter is at least several skin depths) is only
on the outside surface of the braid, not the outside surfaces of all
the wires. The extra loss comes from the necessity of the current
moving from one set of wires to another as the original set goes under
an adjacent group. Surface roughness in itself can significantly
increase RF resistance (cf. Johnson and Graham, _Signal Propagation -
Advanced Black Magic_, Sec. 2.11), but the braid structure increases
the resistance more yet.

I know Tom, W8JI, has measured the impedance of solid strap and
compared it to braided shield, and confirmed that the shield has
substantially higher RF resistance. I don't see measurement results on
his web site, but he briefly discusses the phenomenon at
http://www.w8ji.com/skindepth.htm.

The higher RF resistance of braid is very noticeable in tube power
amplifiers at HF, where the circulating currents are magnified by the Q
of the tank circuit. If the connections between the coil taps and the
bandswitch contacts are made from braid, they can run very hot, while
thin strips of solid copper give no problems at all.

For antennas, the main concern about using braid is corrosion, which
will insulate the strands from one another and greatly increase the RF
resistance. This is why the loss of normal braid-covered coax increases
dramatically if water gets under the jacket.

Ian
Reading the two analysis on braid and RF. It seams to be missinbg
something
or I have things totaly wrong about this. With multiple wires with DC
the
resistance will go down but with RF you have competing SLOW WAVE
phenomina,
bevause each wire is now insulated from each other which prevents wire
hopping.
In other words the corrossion or insulation would DECREASE resistance
would it not? If it is used as a dipole I can see the IMPEDANCE going
up since
the radiator lengths has DOUBLED!
As far as skin depth is concerned I would imagine/think the skin depth
is some what altered
because of the compensating increase in area exposed to RF despite the
fact that
braid presented only half of its useful area!
Could you please point out the errors in my logic?
Regards
Art



However, the original question was about a very short braid connection
snip --

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek- Hide quoted text -

- Show quoted text -

On 3 Nov, 07:02, art wrote:
On 3 Nov, 01:27, Ian White GM3SEK wrote:



Roy Lewallen wrote:
Stefan Wolfe wrote:
Actually braid has muxh less 'resistance' than a flat or round
conducor at RF due to greatly increased surface area and skin effect.

Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those
frequencies that Litz wire can provide some advantage. But it's made of
separately insulated strands. Because of skin effect, the current at HF
(or wherever the wire diameter is at least several skin depths) is only
on the outside surface of the braid, not the outside surfaces of all
the wires. The extra loss comes from the necessity of the current
moving from one set of wires to another as the original set goes under
an adjacent group. Surface roughness in itself can significantly
increase RF resistance (cf. Johnson and Graham, _Signal Propagation -
Advanced Black Magic_, Sec. 2.11), but the braid structure increases
the resistance more yet.

I know Tom, W8JI, has measured the impedance of solid strap and
compared it to braided shield, and confirmed that the shield has
substantially higher RF resistance. I don't see measurement results on
his web site, but he briefly discusses the phenomenon at
http://www.w8ji.com/skindepth.htm.

The higher RF resistance of braid is very noticeable in tube power
amplifiers at HF, where the circulating currents are magnified by the Q
of the tank circuit. If the connections between the coil taps and the
bandswitch contacts are made from braid, they can run very hot, while
thin strips of solid copper give no problems at all.

For antennas, the main concern about using braid is corrosion, which
will insulate the strands from one another and greatly increase the RF
resistance. This is why the loss of normal braid-covered coax increases
dramatically if water gets under the jacket.

Ian
Reading the two analysis on braid and RF. It seams to be missinbg
something
or I have things totaly wrong about this. With multiple wires with DC
the
resistance will go down but with RF you have competing SLOW WAVE
phenomina,
bevause each wire is now insulated from each other which prevents wire
hopping.
In other words the corrossion or insulation would DECREASE resistance
would it not? If it is used as a dipole I can see the IMPEDANCE going
up since
the radiator lengths has DOUBLED!
As far as skin depth is concerned I would imagine/think the skin depth
is some what altered
because of the compensating increase in area exposed to RF despite the
fact that
braid presented only half of its useful area!
Could you please point out the errors in my logic?
Regards
Art



However, the original question was about a very short braid connection
snip --

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek-Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

Ian, to put things more clearly the model I was analysing
was for the frequency rate of 1 Mhz where the analagy was
the use of Litz wire where slow wave contra helices were introduced
and where radiation only oceres in places where the wire is
exposed to the air.
Regards
Art

On 2 Nov, 21:19, Roy Lewallen wrote:
Stefan Wolfe wrote:

Actually braid has muxh less 'resistance' than a flat or round conducor at
RF due to greatly increased surface area and skin effect.

Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those frequencies
that Litz wire can provide some advantage. But it's made of separately
insulated strands. Because of skin effect, the current at HF (or
wherever the wire diameter is at least several skin depths) is only on
the outside surface of the braid, not the outside surfaces of all the
wires. The extra loss comes from the necessity of the current moving
from one set of wires to another as the original set goes under an
adjacent group. Surface roughness in itself can significantly increase
RF resistance (cf. Johnson and Graham, _Signal Propagation - Advanced
Black Magic_, Sec. 2.11), but the braid structure increases the
resistance more yet.

I know Tom, W8JI, has measured the impedance of solid strap and compared
it to braided shield, and confirmed that the shield has substantially
higher RF resistance. I don't see measurement results on his web site,
but he briefly discusses the phenomenon athttp://www.w8ji.com/skindepth.htm.

One other note - for the same surface area, a round conductor has the
least RF resistance of any conductor shape. The reason is that the
current is evenly distributed on the surface. On other shapes it's not,
resulting in higher resistance per unit length.

Roy Lewallen, W7EL

On 2 Nov, 21:19, Roy Lewallen wrote:
Stefan Wolfe wrote:

Actually braid has muxh less 'resistance' than a flat or round conducor at
RF due to greatly increased surface area and skin effect.

Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those frequencies
that Litz wire can provide some advantage. But it's made of separately
insulated strands. Because of skin effect, the current at HF (or
wherever the wire diameter is at least several skin depths) is only on
the outside surface of the braid, not the outside surfaces of all the
wires. The extra loss comes from the necessity of the current moving
from one set of wires to another as the original set goes under an
adjacent group.

I have heard this statement before but consider it just gossip
I see no reason for the so called "necessity of current moving
from one set of wires to another as the original set goes
under an adjacent group". Neither have I come across any proof
of such a thing happening. The impedance on the current carrying
wire is certainly less than that encountered by jumping a "gap"
so there is no "necessity" to creat corona of any sort, especially
when the current has to take a 90 degree turn to get the job done.
I could change my mind if actual proof was presented,
that is if any exists; otherwise I just view such statements
as a propagation of falsehoods while mismanaging a response.
A case in point, a silver contact on a relay after spending
some time in a cardboard package requires in excess of 24 volts
to break down the oxide so imagine what it would take to break
the oxide doiwn of copper!






snip.

Roy Lewallen, W7EL

Art

Thanks to all of the members that contributed !
I appreciate your efforts.
John
VK2KCE