We received reports of high SWR readings with handheld antenna analyzers like MFJ-259C etc.
These antenna analyzers can be overloaded easily when used to measure multiband wire antennas with broadband transformers.
Broadcasting stations nearby overload the analyzer.
A solution for this problem is to ground the analyzer or check your swr readings with the meter at your radio.
MFJ has this new HF Notchfilter for this problems. MFJ 731
All maximum handling powers mentioned are in PEP, SSB and based on 50-50 rate of transmission and receiving.
When transmitting, antenna’s transformer will heat up because of the losses in ferrite. When the temperature of the ferrite reaches it maximum, VSWR will increase until temperature drops again. SSB transmission powers are Peak envelop powers and not comparable with CW and Digital RF powers. For digital and CW power, an average of 35W@50W CW or digital can be compared to 200W PEP SSB.
The principle from a Wide Band HyEndFed Antenna is an AUTOTRANSFORMER.
Itself has enough common mode isolation impedance, so a choke directly under the antenna , can have impact on transformer primary winding and make an impact on antenna tuning.
The most popular, convenient and recommended places are at the bottom of the tower or at the building entrance.
It is also true that if you place the line insulator where the coax enters the house, it suppresses any noise that comes into the shield of the coaxial cable, especially at low frequencies
HyEndFed Multi-band Antennas are using a resonant length of wire placing the current minimum at the feed point.
RF interference in the home is common with any antenna without a good current balun, like dipoles, G5RV etc.
Or when the antenna is very close or installed above the shack.
Avoid the routing of coaxial cable under or parallel to the radiating wire at any antenna.
If not, in that case the antenna radiator will couple the energy to the coaxial shield of the cable and RF is coming into your radio shack.
If a coax cable runs parallel to one leg from a dipole, rf will comes into you radio shack also.