Frequently Asked Questions about EFHW antennas
Do I need a common mode choke at the feed point (transformer)?
No, the transformer itself has enough common mode isolation impedance (5000-19000 Ohms) so additional choke might have an impact on transformer primary winding and make an impact on antenna tuning. We do not recommend any chokes at the feed point especially coiled coax chokes which actually might increase common mode currents on any antenna feedline.
Do I need to add the ground wire from the transformer ground lug to a common ground connection of my station?
The ground lug at the transformer bottom is meant for use with a dedicated ground rod when the transformer is installed up to 10ft above ground. If it is not practical for you to add dedicated ground rod for an EFHW antenna, just omit that connection and ground the coax shield of the feed line to a station ground, entry connector plate or dedicated ground rod with lightning arrestor at the entrance to home/radio room.
What about RFI, Bitey-Microphone and other problems usually associated with a common end fed antennas?
Unlike a commonly used End-fed antennas with 9:1 UNUNs and non-resonant length of wire, EFHW antennas are not prone to high common mode current and problems associated with it mainly due to the high common mode isolation of the transformer (MyAntennas designs) and use of the resonant length of wire placing the current minima at the feed point. RF interference in the home is common with any antenna without proper Current BALUN, like Dipoles, OCF dipole antennas, G5RV, ZS6BKW etc. or when the antenna is very close or installed above radio room like in the attic or routed over the home. Avoid the routing of coaxial cable under or parallel to the radiating wire, in that case, antenna radiator will couple the energy to the coaxial shield of the feedline and cause common mode current on it which will be routed towards your station and manifested as RFI, Bitey mic. noise in speakers etc. Just like any center fed dipole without proper BALUN and coaxial cable swayed on one side, common mode current on a shield of the cable is unavoidable.
Why am I getting high SWR readings on my handheld antenna analyzer?
We have received numerous reports of high SWR readings with handheld antenna analyzers like MFJ-259 etc. It turns out that these antenna analyzers easily can be overloaded when used to measure multiband wire antenna like EFHW fed with broadband RF transformer. It is enough to have a Broadcasting station nearby or HF band (1-30MHz) is wide open producing numerous strong HF signals which overload the analyzer. The common cure is to ground the analyzer or simply check your SWR readings with SWR meter at your radio or use external SWR bridge.
Can I use an antenna tuner to correct elevated SWR on my EFHW antenna?
Yes, Most modern radios have internal antenna tuner and there is nothing wrong to use a tuner to match the output of your radio to an impedance seen at the end of the coaxial cable. EFHW antennas might have elevated SWR readings on some bands due to the different position, height above ground or impact from nearby objects. When SWR is higher than 2:1 or antenna is used on the non-resonant frequency of the wire radiator, power reduction is recommended due to additional mismatch losses and the possibility to overheat the transformer. If the power up to 100W is used there is no need to adjust power on any of our EFHW antenna models. Just press the “TUNE” button on your radio and enjoy.
How the EFHW-8010 antenna radiates, what are radiation patterns per band?
The radiation patterns of an EFHW-8010 antenna varies with frequency and Dick Reid, KK4OBI made a great presentation of 3D and 2D patterns for EFHW-8010 antenna in various configurations. The links to the following configurations are here:
- As Straight wire configuration
- As Inverted V configuration
- As Inverted L configuration
- As Sloping configuration
What wire size I have to chose for my antenna?
We provide two sizes for our EFHW antenna models, the 18 AWG, and 14AWG size wires. The 18ga wire is very stealthy and has a tensile strength of 85 lb (42kg) so you have to make sure that installation involving moving trees has enough sag and not too much counterweight if pulley system is used.
The 14ga wire is much stronger but also heavier and can accumulate more snow or ice. According to the QST article about wire size at this link https://myantennas.com/wp/qst-may-2015-wire-size/ the wire strength really depends mostly on installation and sag amount you are going to leave on the wire.