For the Basic Mini-Whip Antenna 2016 click here
Improved Mini-Whip antenne (10kHz-30 MHz)
New designThere are already countless designs and versions of the MiniWhip antenna. (See my previous article: Basic Mini-Whip Antenna ). However, in a practical implementation, it is not always easy to find a suitable housing for (outdoor) mounting. These are sometimes pots, boxes, tubes and other peculiar housings, which also have to be weatherproof or waterproof.
What if we turn things around? I got the idea to miniaturize the whole thing a bit more, so that we can make use of
a simpler construction, such as e.g. a piece of electrical PVC pipe of 3/4 "(16mm) from the Hobby market, with 2 sealing caps
(for chair legs) top and bottom.
Here the Mini Whip can be fully retracted (Fig. 1).
And so is the Mini-Whip Slimline born .
It is now a lot easier to clamp the whole thing outside, on a metal pole, with standard 3/4 "plastic brackets or clamps.
The concept remains the same, only the PCBs, apart from a few additions, have been adapted to fit directly into the tube.
The inside diameter of the tube is 3/4 "(16mm).
The Antenna plate (Plate) is 100mm x 15mm copper, on single-sided print.
The schematic of the antenna and the power supply:
Click on the figure for a larger view
Just a look at the schedule:
The copper antenna plate (plate) forms a pure capacitance (4 to 6pF) with the surrounding air. This is coupled to the gate of a J310 JFET transistor, with a very small input capacitance (+/- 2 pF). This FET has a high output impedance, and is coupled to an HF transistor, which makes it a low impedance.
The output of this transitor goes with a coupling capacitor to the antenna cable.
The coupling transistor TRW80181 is an old VHF transistor from a cable TV amplifier. This can also be a 2N5309 or a good HF transistor.
The power supply for the antenna is on the same coax cable, and is disconnected in the antenna power line by a choke. On the power supply side, the antenna signal is separated from the power supply with a coupling capacitor and a choke in the power supply line.
The strength of the antenna signal is actually the difference between the signal on the antenna plate and the ground plane on the PCB.
New in this design is the extra attention to interference suppression. Because there is little ground plane on the PCB, it is important that the antenna PCB is grounded at the EARTH POINT, and not the connector. From here there is a separate wire to the metal mast. The metal mast in turn is grounded at the bottom to the ground.
This means that the shielding of the coax cable ends a little before the actual earth point.
The diagram also shows that an additional isolation transformer type TC4-1 is included in the power supply part. In principle, this can be omitted, but there is a chance that the interference level on the antenna will be a lot higher.
The torque capacities are 220nF or 470nF MKM or Ceramic (the value doesn't matter much). Many of these 'special' parts are available from Aliexpress.
The layout of the antenna board (dimensions in mm):
|Fig.1 Antenne + AMP Example of installation in a 3/4 "electricity PVC pipe||
Photo of the PVC tube and the (test) antenna print with a 3/4 "(16mm) electrical PVC tube on top
Layout of the Antenna PCB
Click on the figure for a larger viewThe antenna print (L = 16 cm), fits in a piece of PVC pipe of 3/4 "(16mm), and this is 31 cm long.
Glue a PVC ring of 15 mm (piece 5/8 tube) in the head of the tube, this serves as a distance ring to protect the antenna surface against moisture.
On the right side it is better to omit a BNC connector due to moisture problems. Instead, the cable can be attached to the antenna board, e.g. as shown in the construction drawing, screwed into a solderable PCB terminal block, or soldered directly. Do not forget to insert the supply coax cable through a 6 mm hole in an 18 mm PVC sealing cap first. Then slide a piece of 12cm PVC tube, diam. 5/8 "over the cable. Connect the coax to the PCB.
Slide the assembly into the 3/4 tube at the bottom, up to the thrust ring, in order to clamp the antenna print. Clamp everything with the 18mm cap to seal the tube.
Important: the bottom piece of pipe is now free to use 2 brackets or clamps. It is important to mount the antenna part just ABOVE the metal mast, and not next to it or on the side. The antenna part must be in the open air in order to receive the radio waves from all sides.
The PCB layout and the power PCB (left to the antenna - right to the receiver)
The power supply PCB (dimensions are in mm)
Foto of the power supply PCB
Photo of built-in power supply board
Click on the figure for a larger view
The nutrition PCB fits e.g. in a 19x19 mm piece of square aluminum tube from the hobby market. The length will be slightly different for everyone, depending on the BNC chassis. One end of the tube is folded closed, after sawing off 3 side pieces of the tube (see photo on the right). A hole (12.5 mm) has been drilled on the head of this to allow the BNC chassis wire to protrude outwards.
Important: Where the PCB bottom with copper side enters the tube, slide a 0.3 mm stiff piece of plastic sheet into the tube, so that the copper side cannot make contact with the bottom of the aluminum tube.
If you have mounted an LED, you can drill a 5 mm viewing hole in the tube at the appropriate place at the top.
From the BNC chassis to the antenna, use a long coax cable with a BNC plug on one side. The coaxial cable from the power supply PCB, to the receiver or to an HF Up-Converter, is soldered on the contacts of the power supply PCB.
The other end of the square tube can be closed with a square PVC cap from the hobby market. The cable to be soldered first goes through a drilled hole of 6 mm, in the middle of the PVC cover (see photo). The power supply cable protrudes through a drilled 2.5 mm hole through the cover (see photo).
For a test of this antenna, check my website for the HF Mini Whip Antenna.
Fig.2 Grounding of a MiniWhip
If you want good reception, the following is very important:
This is an OUTDOOR antenna. Do not use them indoors. The electrical interference in the home is simply too great.
Follow exactly the connections in the figure to suppress the antenna. You will see that an extra ground wire is running, directly on the PCB to the antenna mast.
At the bottom of the antenna mast, the mast is grounded again.
Read the article about interference and what to do to avoid it: Interference.
PCB-layout download here : MiniWhip1.2.zip
References (Maybe some are already offline):
Technical pages :
PA3FWM Technotes Grounding
This antenna is great for use with my SDR HF UP-convertor :SDR Up-converter v3.0 XT_125
on1bes at Scarlet.be