Puterea de r.f. in antene
Puterea de r.f. in antene
La majoritatea antenelor produse industrial,firmele respective indica in prospectele lor puterea de r.f. care poate fi "pompata" in ele.Desi am cautat,nu am prea gasit in literatura de domeniu criteriile care conduc la stabilirea-si limitarea-acestei valori.De aceea,m-ar interesa-daca se poate la mare amanunt-criteriile care stabilesc aceasta valoare,altele decit grosimea cablului coaxial de alimentare sau grosimea conductorului antenei propriu-zise sau a conductorului cu care este bobinat torul BAL-UN-ul-ui de adaptare.Cu multumiri! anticipate,Nelu-YO3AOE
Re: Puterea de r.f. in antene
Un raspuns rapid luat de pe un forum
"
There are several factors which limit the power an antenna can
handle. For a wire dipole, you can assume a 50 ohm feedpoint and
calculate the current as a function of the power using Ohm's law.
For example, 100 watts results in around 1.4 amps on the wire.
The ARRL handbook has a table of the maximum current carrying
capacity for different wire sizes - I don't have it handy, but use the
value for free air.
Since most wire antennas will have a radiation resistance higher
than 50 ohms, the wire size you calculate for a dipole will be on the
safe side for longwires, quads, etc. The only exceptions would be
some wire yagis and small loops (and other high-Q antennas),
which can have a much lower radiation resistance. These
antennas need larger conductors for reasonable efficiency, and this
will probably limit the use of small wire for these antennas anyway.
The next thing to consider is the voltage gradiant at the ends of
the antenna. The higher the power (and the higher the Q of the
antenna) the higher the voltage at the ends. There are two
conditions to be concerned with. The first is the insulation between
the end of the wire and surrounding conductors. For most antennas
at the 100 watt level, only a small insulator is needed, and, in most
cases, a length of synthetic rope will suffice. But relying on the
wire insulation alone may no be enough. The other condition is
corona discharge from sharp ends ons the antenna - generally this
is not a problem at amateur power levels, but it can happen,
particularly at high altitudes and with sharp ends on a high-Q
antena. The solution is to use rounded ends or plates on the ends
of the elements to spread out the charge.
One remaining possible power limitation is the dielectric strength
of the insulating materials used to build the antenna. I've heard
stories of loading coils wound on PVC pipe which melt at high
power, for example. (This is highly dependent on the additives
in the specific piece of pipe being used.)
You can estimate the voltage across the feedpoint using the
feedpoint impedance and ohm's law. Again, presuming 100 watts
and a 2000 ohm feedpoint impedance (such as a center fed full
wave dipole) this is several hundred volts, which only requires a
small insulator.
Antenna traps containing a capacitor will have a power limit
which is dependent on the voltage rating of the capacitor (and, in
some cases, the maximum current that the capacitor can pass.)
But if you are just interested in experimenting with wire antenas,
and you aren't trying anything fancy, then the truth is that you can
use quite small wire. I've heard of folks running 100 watts into wire
as small as #36, but one problem with wire that small is that the
birds can't see it in time to avoid it, so it often doesn't last long.
(I suspect that #36 is not rated to carry the calculated 1.4 amps,
but it might work on SSB where the duty cycle is very low.)
Somewhere around #28 is a good practical limit for experimenting,
since the wire becomes more difficult to work with if it gets any
smaller."
"
There are several factors which limit the power an antenna can
handle. For a wire dipole, you can assume a 50 ohm feedpoint and
calculate the current as a function of the power using Ohm's law.
For example, 100 watts results in around 1.4 amps on the wire.
The ARRL handbook has a table of the maximum current carrying
capacity for different wire sizes - I don't have it handy, but use the
value for free air.
Since most wire antennas will have a radiation resistance higher
than 50 ohms, the wire size you calculate for a dipole will be on the
safe side for longwires, quads, etc. The only exceptions would be
some wire yagis and small loops (and other high-Q antennas),
which can have a much lower radiation resistance. These
antennas need larger conductors for reasonable efficiency, and this
will probably limit the use of small wire for these antennas anyway.
The next thing to consider is the voltage gradiant at the ends of
the antenna. The higher the power (and the higher the Q of the
antenna) the higher the voltage at the ends. There are two
conditions to be concerned with. The first is the insulation between
the end of the wire and surrounding conductors. For most antennas
at the 100 watt level, only a small insulator is needed, and, in most
cases, a length of synthetic rope will suffice. But relying on the
wire insulation alone may no be enough. The other condition is
corona discharge from sharp ends ons the antenna - generally this
is not a problem at amateur power levels, but it can happen,
particularly at high altitudes and with sharp ends on a high-Q
antena. The solution is to use rounded ends or plates on the ends
of the elements to spread out the charge.
One remaining possible power limitation is the dielectric strength
of the insulating materials used to build the antenna. I've heard
stories of loading coils wound on PVC pipe which melt at high
power, for example. (This is highly dependent on the additives
in the specific piece of pipe being used.)
You can estimate the voltage across the feedpoint using the
feedpoint impedance and ohm's law. Again, presuming 100 watts
and a 2000 ohm feedpoint impedance (such as a center fed full
wave dipole) this is several hundred volts, which only requires a
small insulator.
Antenna traps containing a capacitor will have a power limit
which is dependent on the voltage rating of the capacitor (and, in
some cases, the maximum current that the capacitor can pass.)
But if you are just interested in experimenting with wire antenas,
and you aren't trying anything fancy, then the truth is that you can
use quite small wire. I've heard of folks running 100 watts into wire
as small as #36, but one problem with wire that small is that the
birds can't see it in time to avoid it, so it often doesn't last long.
(I suspect that #36 is not rated to carry the calculated 1.4 amps,
but it might work on SSB where the duty cycle is very low.)
Somewhere around #28 is a good practical limit for experimenting,
since the wire becomes more difficult to work with if it gets any
smaller."
Re: Puterea de r.f. in antene
Raspunsul respectiv e extrem de superficial din punct de vedere tehnic, autorul ignora efectul de suprafata al conductorilor si nu stapaneste functionarea antenelor, iar concluziile la care a ajuns sunt complet eronate (recomanda fir de 0.32mm pentru 100W ?!!!).
Puterea la care rezista o antena e o limita greu de definit si stabilit, mai ales ca in cazul unui dezacord pot aparea tensiuni/curenti considerabil mai mari decat in cazul unei adaptari optime. Insa, problema cea mai mare nu e puterea maxima la care rezista o antena ci eficienta ei, cu alte cuvinte chiar si la puteri mici ar trebui sa folosim antene care sunt proiectate pentru puteri mari pentru a minimiza pierderile rezistive.
Puterea la care rezista o antena e o limita greu de definit si stabilit, mai ales ca in cazul unui dezacord pot aparea tensiuni/curenti considerabil mai mari decat in cazul unei adaptari optime. Insa, problema cea mai mare nu e puterea maxima la care rezista o antena ci eficienta ei, cu alte cuvinte chiar si la puteri mici ar trebui sa folosim antene care sunt proiectate pentru puteri mari pentru a minimiza pierderile rezistive.
Re: Puterea de r.f. in antene
Corect, nu a luat in calcul efectul pelicular, dar la frecvente mici ( HF ) poti sa il neglijezi sa zicem. Sunt foarte multi factori de luat in calcul si tind sa cred ca la multi producatori puterea maxima admisibila a fost dedusa prin incercari repetate, in varii conditii.
Re: Puterea de r.f. in antene
MAi degraba tine de seriozitatea fabricantului. Spre exemplu negative, la unele verticale itaiene spun ca antenna tine la 300w, dat deja la 150w face probleme
Re: Puterea de r.f. in antene
Cum-in ce fel-se manifesta acele probleme?
Re: Puterea de r.f. in antene
Antena are un SWR pana in 1,2 in benzile dedicate. In clipa in care se ridica puterea peste cam 100 de wati incepe sa aiba un swr de 4-5.
L.E. La aceste antene cuplarea trapurilor se face cu nituri, probabil astea fiind "calcaiul lui Ahile"
L.E. La aceste antene cuplarea trapurilor se face cu nituri, probabil astea fiind "calcaiul lui Ahile"
Re: Puterea de r.f. in antene
Da,niturile sint o pacoste,mai ales cind prin intermediul lor se face si continuitatea electrica,si mai ales daca metalul niturilor este altul decit metalul nituit.De altfel,in electrotehnica unde se cere continuitate electrica nici nu este permisa nituirea cu nituri de alta natura decit metalul nituit,fiindca sub influenta factorilor meteorologici care aduc umezeala,in punctele de nituire se formeaza niste micro-pile electrice,care cu timpul produc electro-coroziune,implicit slabirea-atit electric cit si mecanic a-contactelor,insotita de cresterea zgomotului propriu al antenei, deci numai belele-hi!. Solutia este evitarea-pe cit posibil!-a imbinarilor prin nituire,prin sudura sau lipire,iar cind nu exista aceste posibilitati,protejarea prin lacuire sau vopsire atenta a fiecarei nituiri in parte,in asa fel ca materialul protector sa patrunda cit mai mult si bine-hi! in toate interstitiile imbinarii.Acest din urma aspect este perfect valabil si la imbinarile cu suruburi,care-din punctul de vedere al electro-coroziunii-este foarte asemanatoare-chiar identica!-cu nituirea.Multumesc! pentru raspunsuri si comentarii,cu stima,Nelu-YO3AOE
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