<<

ARDFrx2 with pa0nhc 145MHz three el. antenna.
pa0nhc 20150218  20160407.

ARDFantenna2m
Keep the element- and space dimensions exactly like shown, and use an insulating boom .
Then it will perform optimal without any adjustments.

The receiver was tested during a ARDF event, combined with a new constructed antenna according to my proved design :

I now used cheap and to me available materials : 50mm square ALU pipe and plastic end caps for the receiver housing, a piece of 32mm PVC pipe as boom, three pieces of M6 threaded rod for the center parts of the elements, and six identical pieces spring steel wire of 1.8mm x 385mm (total length) as element end parts, screwed and soldered in M6 "inter connecting nuts". For the receivers housing 50mm round ALU pipe could be used if you do not plan to fit a band spread switch

You naturally can maker you own construction of the antenna and the receivers housing. As long as the boom is made of insulating material and all antenna dimensions are kept exactly like stated, it will work.


The test combination looked like this`.
It is nicely in balance when the hand is between the reflector and the reciever.

In the construction in the photo, the element end parts have all the same length, making assembling easier. All are suitable for all antenna elements. The element center parts have such (different) lengths that, with element ends fitted, they bring the individual elements up to the required total length. Counter nuts prevent element loosening during combat.

    IMPORTANT:
Be careful when being with other people. The thin ends of the elements could harm other people when they ran into them. Turn the antenna elements into a vertical position against your body when passing others. Stay away.

DO NOT use any type of protecting end caps, even plastic ones, nor bend the element ends into a curl, as this WILL introduce a few tenths pico farad extra end capacitances, WILL detune the elements to a lower frequency, and WILL deteriorate the match and directivity of the antenna. How much detuning depends on the extra introduced end capacitances, thus of the end-cap size.

As nobody can tell with a wet finger into the air, if some element protection is fitted, the "lengthening effect" of it should be MEASURED, and the length of all elements corrected (shortened).

As a test, the resonance frequency of a 145MHz test dipole with the given element length should be measured without, and then with end caps. For instance round 5mm balls could be soldered onto the ends. The measured percentage in frequency difference should be used as a shortening percentage to the length of all three antenna elements.

The center insulator was made from a 5cm long piece of broomstick. I drilled the center hole using 12mm and 16mm drills. The inner ends of the M6 threads were filed smaller, and tinned for connecting the coax. Space between the connections was abt. 8mm, and must NOT be subtracted from the total dipole length. NB: it is wise to put two component glue on the thread of the M6 wire rods before you screw them into the wooden center part and fasten the nuts. This prevents unwanted turning of the rods when fitting the element ends. Also water proof the whole wooden center part using polyurethane lacquer. Waterproof the coax ends and the connections, for instance with thermal glue.

Once water is inside the coax, it is useless, as the impedance of the coax is changed dramatically ! You have to dismantle the whole and replace the coax.

As feeder line thin TEFLON (R) coax was used. It was wound through both holes of a (pig-nose type) VHF ferrite core. This prevents currents induced on the outside of the coax mantle to creep to the inside of it via the dipole connection, and then go to the receiver. Otherwise the antenna "skews" and maximum and minimums are shifted aside and are worse.


The thin RG174 or Teflon coax cable has an electrical length of 1/4 lambda. See below.
Observe the black "pigsnose", a ferrite mantle current choke.
There should be one at the other end of the coax too.

REM: inside the receiver housing, there is a identical ferrite core over the coax, preventing mantle currents directly penetrating the receiver input. It prevents skewing of the directivity pattern.

The receivers input is designed for 50 Ohms impedance. Most foxhunt antennas i saw, are designed for best directivity, and do not have exact 50 Ohms impedance, but will present probably a (much) lower value. The length of the coax then is of influence, as it can act as an impedance transformer. But if a half wave length long coax cable is used, this cable does NOT transform the antenna impedance to a still severer mismatch. The receiver then "sees" the unaltered antenna impedance.

        Coax length.
The electrical length of the used coax should be 1/2 wave length (for 144.5MHz).

The total physical coax length becomes :
1/2 wavelength x velocity factor =
((300/144.5)/2 x 0.69) m =
0.716 m for RG174 or
0.781m for Teflon coax.

This total mechanical coax length INCLUDES the coax wound through both ferrite cores, plus the length of the BNC's, plus the length of the coax inside the receiver housing.