-PA0NHC- dipole antenna for 80 to 20 mtrs,
The influence of
temperature changes in Common Mode Chokes.
Antenna (PNG, 800x216x2. 4kB),
NEW choke design (20191207).
For an 80 m band antenna, i only was able to install ONE SINGLE WIRE, with a maximal length of 50 m, running in an angle of about 90 degr. from my house to a tree, .
I constructed an "End Fed" antenna, and later a "Zeppeling" antenna. They radiated relative weak signals, and received loud noises from electronic equipment from the neighborhood, and RF interference from my house. For instance : while CWing the refrigerator was beeping, the radio and TV plopping, the gas heather flame showed to be amplitude modulated.
Do you have the same antenna problems as i had?
You want a coax fed, noise free, RFI free antenna, but only can span a wire at nearly a right angle away from the shack ?
Then read the article below, you find the solution here!
I tried :
- a resonating 40m long end fed wire
- a Zeppelin antenna (40m end fed wire with a 1/4 wave "chicken ladder feeder")
- extra 1/4 wave radials on the roof
- a metal rain gutter as extra "counterpoise". This helped only a bit.
The antenna worked, but:
- I still had a wonderful reception of the wideband noise from the computer regulated gas heather's from all houses around
- TV's, computers etc.
- The flame height of the gas-heater near my shack was still amplitude modulated by my CW,
- Two floors down my wife still heard pocks in the radio
- And the refrigerator-alarm in the kitchen still was still acting as a CW-sounder
Why? The antenna was fed onto one end. and
therefore mechanically Asymmetrical.
It was electrically fully UNbalanced, and used all surrounding metal objects for a RF current return path to the feeder.
The heather, refrigerator, radio and TV acted as a counterpoise.
The solution :
Use a horizontal polarized, pure "symmetrical" antenna.
1. In other words: use an in its center powered dipole.
2. Then one half of the antenna balances the other half.
3. The most active part of the antenna is then around the midpoint, far away from the house.
4. AND the feeder plus the house are in the direction of the MINIMUM field strength of the dipole radiation pattern.
Problem : For a conventional horizontal dipole, the feeder should run down at a right angle to the
center of the antenna.
That was impossible for me.
How to combine the
wanted properties of both :
The zeppelin antenna
The feeder mechanically connected to one end of the antenna,
and running in the same direction as the radaitor
The dipole antenna
The feeder electrically connected at the center of the antenna.
combination of these demands lead to a
This is a vertical polarized omni directional VHF antenna, made of pipes and rods. The antenna stand pipe acts as coax feeder and is connected to the open center of the 1/2 wave long radiator. The lower part of the radiator acts as a BALUN (a so called "bazooka"), and decouples the feeder from the radiator.
The feeder runs to one end of the radiator, but
feeds the center of the radiator. As a result, the feeder is in line with the
radiator, in the minimum of its radiation pattern, and the antenna is
electrically symmetrical. This was exactly what i wanted. So
"simply" turn the coaxial dipole 90 degr. to the horizontal position, make it
resonant on 80m, and you have in principle the
antenna described here below.
Such an antenna proved to work well between 1.8 MHz and 14 MHz, needing a tuner outside 80m.
How the coax choke works.
contains TWO skin layers which (for high frequencies) are separated
from each other :
1. One skin layer ("Skin2") is on the INside of the screening. This internal circuit is called the Differential Mode circuit. It only carries energy which runs to and from the antenna. Skin2 does not interact with the outside world.
2. On the OUTside of the screening a second skin layer "SKIN1" exists.
This outside circuit is called the Common Mode circuit.
It interacts with the outside world and receives signals and noises from the surrounding. It therefore carries noise currents, which will run towards the antenna and the transceiver. It also "see" ferrite cores which are placed over the coax.
Well constructed Ferrite coils can couple very high impedances in series with this common mode circuit, thereby vastly reducing common mode currents.
At abt. 20m distance from the end of the coax
part, the high impedance of such a Common
Mode Choke (CMC) is coupled
in SERIES with Skin1.
This results in an electrically "ending" Skin1 at that point. The outer skin of the last 20m coax now is a 1/4 wave long dipole half.
That part carries two RF-currents :
- One non-radiating differential mode current inside the coax towards the feed point (middle) of the antenna.
- One radiating common mode current on the outside of the screening, running from the feed point backwards to the antenna choke.
The end impedance of a dipole can be several kilo Ohms. The impedance of the choke must be higher and resistive of nature to prevent detuning of the antenna. The antenna choke must therefore be constructed using #31 ferrite material cores, with ample turns, and two staggered-tuned CMCs connected in series.
At the feed point, the coax inner conductor is connected to the 20m long stainless steel wire, which radiates the supplied energy.
The coax outer conductor is kept isolated from the inner conductor, and connected nowhere. The current at the screening skin layer (Skin2) differential mode circuit will jump to the outside common mode circuit, where it is radiated..
As the feedpoint is electrically open, the dipole center construction is very light, needs no housing, but static charges onto the wire part must be discharged to ground elsewhere. See water proofing.
On 80m the 40m long antenna is a half wave long, has a low VSWR into 50 Ohms, and probably needs no extra matching.
On 40m the antenna is a full wave long, and has a high feed point impedance (over 1 kOhm). The VSWR in the 50 ohms coax cable then becomes very high. But the cable losses are still low, due to the relatively low operating frequency. But the coax must be able to carry high voltages.
At the transmitter side of the coax cable, the impedance can be anything then, depending on the total length of the coax cable and the working frequency.
TIP : when the electrical length (L/0.66 or L/0.72 for PTFE) of the differtial mode circuit of the coax cable is an odd number of 1/4 wavelengths, the cable acts as an impedance-inverter. The impedance at the transmitter side of the cable is then very low if the feed point impedance is high..
Pre matching can be done at the transmitter side of the coax, until the VSWR <= 3. Avoid the use of ordinary toroid ferrite matching transformers. They can saturate and cause severe losses and harmonics if their parallel impedance it not high enough.
If you want some form of pre-matching, i suggest a PI-filter type matcher.
When the TOTAL electrical length of the coax (antenna + feeder) for the used frequency is an odd number of 1/4 wave lengths, the coax line can act as an impedance transformer. A high antenna feed point impedance will be transformed to very low impedance at the transmitter site and vice versa. At 3.65 MHz the antenna shows an impedance near 50 Ohms, causing the coax length to be UNcritical for the 80m band. If at other bands than 80 transmitter matching matching shows to be difficult, the feeder impedance at the transmitter site can be changed by extending the coax feeder a bit.
WARNING: do not try to match a VSRW higher than 3 using more than e few watts
with a build-in automatic tuner in your transceiver
The coax feed line of this system
is insensitive to antenna radiated energy.
The feed line of this antenna runs in the MINIMUM of the antenna radiation pattern, and at a right angle to the building. The building therefore is also in the MINIMUM of the antenna radiation pattern. Resulting in no RF power on the outside of the coax feeder and NO RF in the house.
If there is not
enough room to stretch a 40m long wire.
Still use a 40mtrs long dipole. Bend both ends of the radiating part of the antenna at an angle of 90 degr. The antenna becomes (seen on top) Z-shaped. It still matches at 80mtrs. As the biggest part of the RF-current is running from the center of the antenna, this trick it has little negative effect on the antenna efficiency.
If you cannot use the full 40m
Just install the max length you can. But keep the radiating part of the antenna symmetrical.
In other words: keep equal length of both dipole halves. The antenna will work less good on lower frequencies, but will still work and will be symmetrical, with all the positive effects. You have to use a tuner anyhow. And remember : high SWR on 80m does not cause much cable losses.
Don't worry for not ideal SWR at 1.8 / 3.65 / 7.1 MHz..
Stick to the proposed materials and construction. If you know better solutions, pse let me know.
Use MILSPEC. cable, for instance RG58 CU, for up to 100 watts transmitter power.
For higher power levels, especially on 7 or 14 MHz, you can use AIRCELL coax or RG214. Also use RG300 PTFE cable for the choke. The antenna will become heavier, so you need a thicker supporting rope, and have to apply much more stretch-tension.
Use ONLY black cable ties of the best quality. Dont be stupid as i was experimenting with cheaper types.
They should not weaken in UV sunlight, should not slide loose, and should be strong.
Use small ones for the coax, and larger ones for the choke.
Use only pre-streched polyester flags line rope of 4-6 mm dia.
Do NOT use nylon! (this lengthens when loaded, and hardens due to sunlight-UV).
Fixating the coax to the rope.
Over the whole length of the rope, make every 50cm a little opening, by pushing a sharp pointed tool between the strands (do not damage the rope). It helps, when the rope is pushed a bit together in the length direction.
Then put a little black cable tie completely through the hole.
Then span the rope between 2 points with good tension (a bit more tension then later will be used when the antenna is installed in place). Start binding the coax to the rope at the antenna feed point first. At the feed point leave enough coax length with some slack, to connect the inner conductor to the wire clamp..
Wind each cable tie fully once around the coax, then close the cable tie just enough. The coax must NOT be strangled, but also may not easily be
See to it that, between to cable ties, the coax is NOT stretched, but just hangs a little slack. You must be able to put your thumb between the coax and the rope.
This prevents damage to the coax cable when pulling forces stretch the antenna.
Connecting at the feed point.
For easy handling, leave enough unshielded length (20 cm) of the coax inner insulation + conductor.
Before crimping the hose, put Vaseline or Tectyl ML over the whole connection to waterproof it.
The inner conductor of the coax cable will be electrically connected to the steel wire part of the antenna by means of a solder-eye .
Connect the solder eye of the inner coax conductor to one nut of the wire clamp on the rope end.
See to it, that there is no stretch onto the coax!
Weatherproofing the coax
Waterproof all connections before and after. I used a spray-can TECTYL ML (thin, penetrating, water-repellant, non hardening, anti corrosion car chassis wax). Seal the center connection using thick black heath crimp hose, if possible with heath-melting resin inside.
If after crimping the ends of the crimp hose should not fully close :
Immediately before heath crimping, fill the open ends of the crimp hose with thermal glue (glue pistol).
Then immediately crimp the connection. The glue will melt together with the hose and the cable and close the connection for humidity.
Spray the ends with Tectyl ML.
Installing the choke.
Using big FairRite #31 ferrite "SnapIts" cable clamps is the easiest way for installing the choke.
a distance of abt. 18.3m away from the antenna feedpoitn, wind the coax the
specified number of turns through the open clamps. After closing the clamps
CHECK that both halves of each clamp can close perfectly. The
clamps should be able to be turned around the cable windings without any force.
If not, remove one turn. Secure the clamp closure using a ty-wrap.
Fix each clamp to the rope by fixing the coax near the clamps to the rope.
Your antenna is now ready to install at an height between 10m and 20m over ground.
The antenna radiates and receives the most near the feed point, that is the center of the radiating part ! That is the MIDPOINT of the antenna between the far end and the choke.
So keep this area as far away as possible from surroundings and noise-radiating sources. Keep that dipole-part straight and high.
Run, if possible, the part of the coax cable running between your transceiver, and the point where the dipole is mechanically connected to the house, at 90 degr. to the dipole´s length axis.
Protect the coax, your transceiver and yourself by using a coaxial lightning-arrestor in the feed line and a good NOISEFREE safety-earth. Static's can build up easily, as there is NO connection between inner and outer conductors of the feed cable.
READ this pse.
Pse let me know, if you did build the end-fed dipole. I am interested in your results.