<<

-PA0NHC- dipole antenna for 80 to 20 mtrs,
with coax feed on one -END-

Published in Dutch in " ELECTRON" november 1990 pages 589 to 591.
Update: 2004 10 29 / 20110309 / 20150216t

You can separately load and print hi-res sketches of:

antenna (PNG, 800x216x2. 4kB),

choke (PNG, 567x200x2. 5kB).

For an 80 mtrs antenna, i only was able to install ONE SINGLE WIRE, running in an angle of about 90 degr. from my
house to a tree, for maximal 50 m length.

This antenna caused loud noise reception from electronic equipment around, RF interference in my house and relative weak signals.

Do you have the same antenna problem as i had?
Want a coax fed, noise free, RFI free antenna?

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")
- extra 1/4 wave radials on the roof
- a metal rain gutter as extra "counterpoise". This helped a bit.

    Results: It 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 modulated by my CW,
- two floors down my wife still heard pocks in the radio
- and the refrigerator-alarm in the kitchen still was acting as a CW-sounder

Why? The antenna was fed onto one end. It was electrically not balanced, and had to use all surrounding metal objects for a RF return path.


    An electrical solution:

Use a horizontal polarized pure "symmetrical" antenna. In other words: use a dipole. One half of the antenna then balances the other half. The active part of the antenna is around the midpoint of the antenna, far away from the house.

But the feeder should run at a right angle to the antenna, from the transceiver to the feed point of the dipole. That was impossible for me.

        So how to combine the nice properties of both
zeppelin-antenna (feeder mechanically connected at the end of the wire and running in the same direction) and
dipole (feeder electrically connected at the center of the antenna)? 

    The mechanical solution is:
The coaxial dipole. This vertical polarized omni directional antenna is for VHF made of pipes and rods. The coax feeder runs through the antennas stand pipe. The lower half of the radiator acts as a "bazooka", and decouples the antenna from the stand pipe and the coax.

As a result, the antenna feed point is balanced ("symmetrical") without the use of a transformer. The feeder cannot pickup HF-energy and cannot induce noises to the antenna. Because it runs away at the centerline of the radiator.

This was exactly what i wanted. So turn the coaxial dipole 90 degr. to the horizontal position, make it resonat, and you have in principle the antenna described here under.

    Further development:
But how to construct a 40m long horizontal "Coaxial dipole"? It has to be done with light weight coax cable.

The coax feeder runs upto the feed point (mid) of the antenna The inner surface of the coax cables screening acts as part of the coaxial feed line. The outer surface of the last 1/4 long piece of the coax screening act as one radiating dipole-half.

On the last 1/4 piece coax two RF-currents are flowing :
one on the inside of the screening towards the feed point
the second on the outside of the screening from the feed point to the end of the 1/4 wave radiator.

Therefore, for the last 1/4 wave of the coax, good coax should be used if mechanical forces allow it, for instance RG214 with double, silver plated screening.

But as the coax part of the radiator must be limited to 1/4 wave length, at 1/4 wave distance a choke is inserted for currents on the outside of the coax. It is in de shape of the feeder coax wound into a coil. This coil is not of influence to the internal field of the coax, only to the external field. Three millimeter or five millimeter dia. coax is useable for winding the choke.

REM: If the antenna is resonant on the low frequency short wave bands 80m or 40m, thin coax only does cause little losses.
At 21MHz and 28MHz you could use low loss coax, as a choke with less turns is needed.

The end impedance of a dipole can be several kilo Ohms. The impedance of the choke must therefore be high.

For 7MHz and higher this choke can be wound on three big ferrite cores in series, each made of two stacked Amidon FT140-61 cores (see picture). Or over a ferrite rod (Amidon R61-050-400, u=125).
Its self inductance should be at least 50uH, and its Self Resonance Frequency must be above 14MHz if you want it to tune for 14MHz too.

For 3.5MHz it is wound over a bundle of three or four ferrite rods (Amidon R61-050-400, u=125).
The self inductance should be at least 100uH, and its Self Resonance Frequency above 7MHz if you want it to tune at 7MHz too.

Again: Care must be taken that the impedance of the coil is high enough on the lowest working frequency, AND the self resonance frequency of the choke coil is above the highest working frequency.

The coax cable is supported by an insulating rope, which runs all the way from the house to the feed point.

Sketch of PA0NHC end fed coaxial dipole for 80-20 mtrs.

    Feed point.
At the feed point, the outer conductor of the coax cable (screening) is kept isolated from the inner conductor, and connected nowhere.
It just ends there.

The other half of the dipole is made of stainless steel wire, electrically connected to the inner conductor of the coax. Mechanically connected to a 6mm dia. polyester flags line rope, by means of a stainless steel wire clamp. 

Because the choke has some stray capacitance, the coaxial part of the 80m band dipole is shortened a bit (ca. 1m20cm), to compensate for the lengthening effect of the choke on that dipole part.
For resonance at 3.65 MHz, the choke could be placed at 18.3m from the feed point. The wire part could be 19.5m long.

As there is NO transformer or coil connected across the feed point, it cannot shunt the feed point at low frequencies. When the feed point has an high impedance, it cannot saturate nor generate harmonics.

Consequently there is no need for a heavy connecting box at the feed point.

See water proofing.

    Impedance.
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 total length 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.

    Matching:

Pre matching can be done at the transmitter side of the coax, until the VSWR <= 3. Avoid the use of ordinary toroidal ferrite matching transformers. They easily can saturate and cause severe losses and harmonics. If you want some form of pre-matching, do this with a non-saturating device. This can be done with a fixed PI-filter type matcher, or with a special kind of transformer, see TELETRIX at the end of this page. 

 

WARNING: With a build-in automatic tuner from your transceiver, do not try to match a VSRW higher than 3 using more than e few watts This could burn out ceramic high voltage capacitors and relays !

    If there is not quite enough room to stretch 40m long wire, a solution :

still use 40mtrs wire, and bend both ends of the radiating part of the antenna at an angle of abt. 90 degr. The antenna becomes (seen on top) Z-shaped. It still matches at 80mtrs. Because the most of the RF-current is running near the center, the feedpoint of the antenna, it has little effect on the efficiency.

If you cannot use the full wire length, just install the max length you can.
But keep the radiating part of the antenna symmetrical. In other words: keep the length of both halves of the dipole equal. 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.

    Construction-details.

Stick to the proposed materials and construction. If you know better solutions, pse let me know.

    Feeder-coax:

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. This will give also less insertion-losses. Also use thicker PTFE cable for the choke. The antenna will be heavier, so you need a thicker supporting rope, and have to apply much more stretch-tension.

       Cable ties.
Use ONLY black cable ties of the best quality.
Buy a bag of 50 pieces. I suggest you use the brand TYWRAP.  They do not weaken in UV sunlight, nor slide loose, and are strong. Use small ones for the coax, and  larger ones for the choke. Be not so stupid to experiment with cheaper types as i was.

    Weatherproofing the coax connection:
I did not try to use self vulcanizing tape. Use thick black crimp hose, if possible with heath-melting resin inside. If only normal crimp hose is available, use a spray-can TECTYL ML (thin, penetrating, water-repellant, non hardening wax) to waterproof all connections before and after crimping the connection.

Practice the following procedure before real use. When after crimping the ends of the crimp hose can not fully close, fill before crimpng the open ends with thermal glue (glue pistol) and immediately crimp then. The glue will melt together with the hose and the cable.

    Choke:
Only use PTFE-coax for the chocke, this withstands soldering and higher voltages.

Method 1: Use on 4 toroides u=125 36x23x15mm, Philips 4C6 or 4C65 (pink color) or equivalent.
Connect all coils in series (SEE DRAWING for how to wind).
Best is to use one piece of coax for the whole 4-core coils.
Wind each toroide with 2x7 turns (see drawing!).
Fix the coils with small cable ties on a thin fiberglass or plexy glass plate.
Then insert the whole assembly in a piece of thin PPC plastic sewer-pipe.
Put a little stiff plastic foam between coils and pipe to prevent rattling.
Close the pipe with watertight, UV resistant  lids.
Drill 2 small holes at the lowest points, to allow some ventilation and draining.

Coax-choke for PA0NHC end fed dipole with toroides.

 In the future i will develop a new choke construction.

The toroids also could be installed in a watertight plastic box with water tight feed troughs for the coax. Drill at the lowest point one small draining-hole.

Method 2: Also is usable a coax coil wound around some ferrite rods. Rods cannot be easily saturated. Best should be to use rods with u=125, Amidon material 61. This has optimum properties for the frequency range of use.

But you can also construct a "rod" yourself from a lot of small toroids with u=125, glued on top of each other with 10 second glue.

    Divide and rule!
You can use coils of different inductances and self resonance's in series (like the plate-chokes of SW-pa's did in the old days). Check the self resonance of each separate coil with a dip meter. These resonance's must lay outside and above the used amateur bands. Probable the best solution should be: to divide one big coil in several parts with different parallel resonance's, by means of extra space between the coil parts. The total reactance must be high enough for the lowest working frequency. Wind the coils, but each time allow a separation by winding one turn with a winding space equal to the cable thickness.

I tested such a coil set successfully.

    The rope:
Use only pre-streched polyester flags line rope of 4-6 mm dia. Do NOT use nylon! (this lengthens when loaded, and it deteriorates due to sunlight - UV).

    Waterproofing the coax-end at the feed point:
For easy handling, leave enough free length (20 cm) of the coax inner insulation + conductor.
Fully encapsulate the end of the coax with crimp hose and Tectyl ML (water protection).
The inner conductor of the coax cable will later be connected to the steel wire by means of an solder-eye .

    To fix 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 shifted.
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 the choke:
Cut the coax at the point where the choke must come. For resonance at 3.65 MHz, the choke could be placed at 18.3m from the feed point. Mount the choke firmly to the rope by means of ty-wraps. Connect the coax with some slack to the choke. The connections can be made by soldering and crimphose, or with BNC male+female connectors.

    When Soldering:
first slip the necessary insulations over the coax. Use heat-resistant glass hose for the inner conductor, and crimp hose for the outside.

On each coax part, widen and pull back the screening a few cm.
Solder the inner conductors together.
Slip the glass hose back over the soldered connection.
Then shift the screenings over the glass hose onto each other, and solder the coax-screenings together. Check for short-circuit in the coax.
Before slipping the crimp hose over the completed connection, waterproof each completed connection with a little TECTYL ML.
Crimp and then waterproof again with TECTYL ML.

You can also use a piece of thin copper foil to connect the screenings together.

    When using BNC connectors :
First slip the crimp hose over the connectors.
Waterproof the connectors with a bit TECTYL ML.
    Quickly :
Fill both ends of the crimp hose with thermal glue (HOT glue pistol), and crimp immediately.

    Final assembly at the feed point:
At the feed point, connect the inner conductor of the coax cable to one nut of the wire clamp on the rope end. Use a small soldering eye.
See to it, that there is no stretch on the coax!
Waterproof the connection with TECTYL ML.
Slip the crimp hose over the wire clamp.
    Quicly :
Fill ends with HOT thermal glue and crimp immediately.
Again waterproof with TECTYL ML.   

Your antenna is now ready to install at an height between 10m and 20m over ground. 

    Remember:
The antenna radiates and receives the most near the feed point! 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 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.

 


You can separately load and print hi-res sketches of:

antenna (PNG, 800x216x2. 4kB),

choke (PNG, 567x200x2. 5kB).

Eight PDF pages with data of Philips 4C6 ferrite material :

1 2 3 4 5 6 7 8

 


    Where to get parts:

TELETRIX NEDERLAND BV. is a small but quality firm.
Phone: +31 115 613570  Fax: +31 115 615659 .

Owner PA0LB developed military grade very wide bandwidth "Guanella" type antenna transformers for a variety of impedance ratios and power levels. Tested in climate rooms. Also suitable for wideband matching of magnetic loop antennas.

He also can deliver complete wire antenna's, stainless steel wire and hardware, toroide cores, PTFE coax. Ask for a brochure.