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Author Topic: high power Mobile antenna and fundamentals  (Read 1067 times)

Posts: 39

« on: March 06, 2012, 09:23:46 AM »

Ok, i love this eham forums,
i have a MFJ1432 , which is identical to the Diamond SG7900NMO , now the antenna is stamped "200 watts max" but the website says 150 .. diamond wensite also states 150 watts, HOWEVER, it also says "150 watts sum of both bands simutaniously" , do you mean to tell me i have been using a 75 watt antenna with a 160 watt mirage amplifier? I just emailed Diamond to see what they say, but what are the negative effects of overloading an antenna, EXCEPT damage to the system (amplifier), would i have reduced TX range, etc... i really do well with this setup, significant notice with amp on and off, but i want a better antenna, and i dont mind length, thisone is 62 inches, 7/8 wave.. i have seen an antenna that is 96 inches, monoband 2 meter, lots of gain... how do i know if a 1/4 wave antenna that is rated at 200 watts will outperform this antenna because im overloading it.. i am using LMR400 hard mounted to roof on NMO mount.

Posts: 17482

« Reply #1 on: March 06, 2012, 10:36:40 AM »

Quote from: KI4MQJ
...HOWEVER, it also says "150 watts sum of both bands simutaniously" , do you mean to tell me i have been using a 75 watt antenna with a 160 watt mirage amplifier? ...

No, the antenna is rated at 150 watts maximum.  But if you run 100W on 2m and 50W on 70cm,
that totals to 150 watts.  You can't run full rated power on both bands at the same time.

What happens if you exceed the power rating?  Three possibilities:  either something overheats
(which is a function of duty cycle), something pops (which is immediate damage) or it arcs
somewhere, in which case it may still be fine at lower power.  If overheating causes a plastic
insulator to soften, the damage may happen over time depending on how long your transmissions
are.  A popped capacitor will throw the SWR off immediately, and need to be replaced.  An arc
between two points inside the housing may only show a high SWR when the power level is high
enough, but work fine at lower powers.

If it was originally rated at 200W but has been lowered to 150W by both companies, it probably
was marginal at 200W for some reason.  But without researching comments by hams that had
problems with that particular antenna, you can't know which symptom they encountered.
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