Frequently Asked Questions about EFHW antennas
We receive hundreds of messages every year claiming that something is wrong with our EFHW transformers. The common complaint is about the DEAD SHORT circuit across the SO-239 connector measured with a multimeter. In fact, it MUST be a DEAD SHORT between all connections on the box since primary and secondary winding share the common connection at the shield of the SO-239 connector. The multimeter measures the connectivity at DC with a 9-Volt battery inside, and of course, it will be a short circuit if you look at the schematics of the transformer in the following image.
In order to test the RF transformer, You need an RF source like a Handheld analyzer and resistor of 2500-3000 Ohms connected to antenna wire connection and another end to ground lug/ Shield of SO-239 connector. Remember wire should be disconnected and placed further from the transformer. Then sweep the SWR from 3-30MHz and depending on the type of resistor and its leads, the curve should be in form of a shallow W shape with VSWR peaks no higher than 2:1, just like in the following image
Yes but only with low power like 200W maximum. The SWR is high (over 3:1 ) and a reflection of 25% or more will dissipate on the transformer core which can lead to the destruction of the same. If you prefer a 3.8-4MHz sideband (SSB) operation, get an EFHW-7510-2K+ model which has low SWR on 75M and maintains the low SWR on other bands just like the EFHW-8010 model does.
Here is the list of possible problems you might have experienced with EFHW antenna models:
1. Your SWR is elevated upon the first installation.
*Check the Cables, connectors, lightning/surge arrester.
*Make sure antenna is not touching objects or being very close to objects like the roof, siding, etc.
*Never have a wire going over the tree limb without proper separation with at least rope or insulator.
*Never run wire parallel to the tower or metal pole at close proximity, three feet (1 Meter) is minimum but still expect interaction.
2. Your SWR is creeping up with power applied in longer transmissions like digital (FT8, RTTY, etc), Long SSB transmission, or CW high speed chatting.
* The EFHW transformers covering 3-30 MHz are impossible to be made without Insertion loss which is averaging at 0.6 dB on our models. That loss in combination with elevated SWR like 3-4:1 on ie 3.9 MHz used with power and antenna tuner after the amplifier, when used with our 8010 model, will overheat the ferrite core and SWR will slowly “creep up”. NEVER use power and tuner with SWRs above 2:1. Reducing power to a 250-500W is OK depending on the Mode and duration of transmission used. Our models of 1kW and 2kW can easily be used at those power levels in short transmissions like calling the rare DX but not for a prolonged period of time.
3. Your SWRs suddenly are changed after months or years of installation.
* Common mistake is over the tension of wire with supports like moving trees which will stretch any copper wire and shift the first resonance of 3.55 MHz for 8010 model or 7.1 MHz for 4010 model, below bands like 3.3-3.4 MHz and 6.8-7 MHz. That will result in a shift of resonances and minimum SWR on all bands above.
* You have installed another antenna in close proximity (like Reactive Near Field distance of 0.159 wavelengths) to the antenna wire which will result in mutual coupling and de-tuning of both antennas.
*The proximity of the antenna to home and coupling of RF to miles of wires inside your home is also a major reason for RFI (Interference to appliances and your equipment like transceivers, modems, PCs, etc)
4. Your wire broke
*It is always a sign of over tension even with the use of pulley/counterweight or bungee cords, springs, or similar. The Moving trees will easily exert hundreds of lb of tension and wire with 85lb (16ga) or 200lb (14ga) of tensile strength will break or simply stretch by several feet. Abrasion of wire if it is routed thru some insulator with sharper edges is also things to avoid. As detailed in this QST article about wire size, wire strength ultimately depends largely on the installation and sag amount you are going to leave on the wire.
5. None of the above, let us know.
We provide two sizes for our EFHW antenna models, the 16AWG XLPE for 1K models, and 14AWG XLPE for 2K models size wires. The 16ga wire is easily concealable (For any HoA concerns) and has a tensile strength of ~100lb (45kg).
Always make sure that any installation involving moving trees has enough sag and not too much counterweight if a pulley system is used otherwise the flex in a swaying tree limb may snap your wire!
The 14ga wire is considerably stronger but also heavier with a larger surface area which can accumulate more snow or ice. As detailed in this QST article about wire size, wire strength ultimately depends largely on the installation and sag amount you are going to leave on the wire.
The radiation patterns of an EFHW-8010 antenna varies with frequency; Dick Reid, KK4OBI provides a great presentation of 3D and 2D patterns for EFHW-8010 antenna in various configurations. The links to the following configurations are here:
Yes, Most modern radios have an 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.
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.
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.
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 10 feet (3 meters) above ground. If it is not practical for you to add a 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 pass-thru SO239 or with lightning arrestor at the entrance to the home/radio room. The grounding of all coaxial cable shields at the entrance to the home is required by National Electrical Code NEC 810
Also, do not listen to internet “experts” saying that some form of hanging counterpoise wire of 0.05 wavelength should be added to the ground connection of the transformer box. It is useless and all you need is to provide good grounding of the coaxial cable shield anywhere along the line between the transformer and entrance of the cable to your home.
The EFHW antenna will work without any grounding too but it is recommended for protection of your equipment.
As original article of an inventor Josef Fuchs, published in July 1928 QST magazine says this:
“The unilateral connection of the aerial to an intermediate circuit as seen in Fig.1 has the advantage of working without ground or counterpoise…”
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. BUT we strongly suggest installing the CMC at the entrance of your cable to the home in order to prevent any induced RF or common-mode noise signals to enter the home and interfere with your station and appliances. Note that all cables entering homes must be grounded before entering the home and CMC should be installed between that grounding point and antenna. See Grounding links in our tech library.
No, You can use any length of 50 Ohms Coaxial cables such as RG8x, RG8, RG213, LMR-400, DXE-400Max or similar. The feedline length does not change the electrical length of the antenna nor change the resonant points of the antenna. The minimum SWR points remain on the same frequency regardless of feedline length.