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G8MNY > TECH 04.11.08 09:26l 86 Lines 3685 Bytes #999 (0) @ WW
BID : 7430_GB7CIP
Read: GUEST
Subj: Coax Feeder Tests
Path: DB0FHN<DB0FOR<DB0SIF<DB0GV<DB0LJ<DB0RES<ON0BEL<ZS0MEE<GB7CIP
Sent: 081104/0045Z @:GB7CIP.#32.GBR.EU #:7430 [Caterham] $:7430_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By John G8MNY (New Jul 08)
When putting up a new aerial system (fixed, /P or /M) it is generally worth
while testing the feeder. RF SWR testing seems the best as far as the Tx goes,
but is not always fool proof.
DC TEST
With an Ohm meter it is quite simple to measure the coax & if there are no
copper whiskers across the Tx or Aerial coax ends an open circuit will be seen.
Tx End \³/
Meter > ͵±±===========================±±ÆÍ Ù
Plug Coax Plug Aerial
But this is not the most efficient DC test, by putting a dummy load at one end
e.g. instead of the aerial, Short, Open circuits & then RF tests can be done.
Tx End Aerial End
Meter > ͵±±===========================±±ÆÍ 50ê Load
Plug Coax Plug
Here the meter should reads 50ê + cable DC resistance.
RF TESTING
Aerial End
Tx End
SWR Bridge> ͵±±===========================±±ÆÍ 50ê Load
Plug Coax Plug
After the DC test, this RF test will reveal if there is problem that will
affect the Tx.
Now putting the aerial in should give the same results.
Tx End \³/
SWR Bridge> ͵±±============================±±ÆÍÙAerial
It is worth noting if the aerial is a DC short or not, for future testing.
It is possible to get a good DC & RF SWR indication on a faulty installation
where say the braid is bypassed by other earths & the coax outer becomes the
aerial.
CONNECTIONS
The most common is PL259, with its 4mm wander plug centre it is rated at 500V
& 10A, it is hardy plug usable to 200MHz (400W). The Z is not 50ê on the cheap
versions & generally the mismatch length is so short compared to a 1/4 wave it
is quite irrelevant as is the loss, but fully waterproof 50ê version are made
for a price. To waterproof the cheap ones it is best to fill them with grease
(vaceline) & then tape over the outside with amalgamating tape.
Making a good shield connection in some designs can be difficult & some have
white nylon insulators that melt easily, while others have brown ones that
burn & char. Small cable bore plugs or adaptors are needed for thin coax. There
are in line female SO239 made, if you can find them, otherwise you need barrel
joiners.
N connectors are used above 200MHz, these are made to be 50ê & 75ê, & are
fairly rugged, but the thin centre pin & socket & small 6mm surrounding shield
connection (not the screw up bit) must give good RF connection to maintain 50ê
& low losses. The thin pin is not rated to carry heavy current like the 259 so
high SWR & high power, can potentially damage it. There are many designs of
plug/socket with loads of fiddly bits to put on the coax in the right order,
but the principle is to keep the Z constant, so the outer connection is made
close to the front of the plug as possible, until the centre conductor size
alters. Be aware the waterproof seal not only keeps water out but in too!
BNC connectors use the same small 50 or 75ê pin N plug system but with small
bayonet locking arrangement. Normally made for 5mm & 7mm coax, but larger cable
types are made.
TNC connectors are as BNC, except have a smaller screw outer than the N type.
All plug into each other to make a good RF link when the outer screw/bayonet/
larger screw part does not foul.
SMA/SMB/SMC series connecters are use on UHF small kit, my advice is to get a
wander lead to a sensible plug/socket.
Why don't U send an interesting bud?
73 de John G8MNY @ GB7CIP
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