Saturday 12 February 2011

70Mhz (4 metre) Delta Loop. A simple antenna for the four metre amateur radio band. By Stephen Studdart, GW7AAV.



  

Having just obtained an AKD 4001 and after the success of the 6 metre Delta Loop, which included working most of Europe in a day during a recent lift, I decided to have a go at one for 4 metres.
Although I would only have FM on 70Mhz I thought that making another horizontally polarised Delta Loop made sense for a number of reasons, first it was simpler to mount and secondly it would give me chance to see if nesting the loops caused any problems.
4m Delta Loop.

The Delta loop has an impedance of about 100 ohms and to match it we use a quarter wave of 75 ohm coax in series to with the 50 ohm feeder. What might be confusing to those new to antenna making is that we need to compensate for the velocity factor of the cable we are using. If you know what cable you are using there are sources on the Internet (such as the manufacturers web site) that will give you this specification. I used a high quality air core satellite TV coax with a good braid and foil shield. The velocity factor for this cable was 66 therefore… a quarter wave on 4m is 1m and 66% of 1m is 66cm.
This matching section is critical and you need to have 66 cm of shielded section. The trick is to cut 72 cm of 75 ohm coax and then strip back 3 cm either end to make the tails which are tinned with solder and screwed into the block connectors.
4m Delta loop inside 6m Delta Loop.

Well this one appears to be working, and to my surprise the SWR was 1.1 to 1. Right first time again so I am very pleased. Nesting the two Delta loops seems to work okay with a slight but very acceptable rise in SWR on the 6m loop but no change on four metres.
By moving the feed point to a top corner and taping the apex of the triangle to the mast I have tested the antenna vertically polarised, which for FM works a lot better.
My findings on 4m lead me to think that although this good antenna for portable work and it works well from high locations to be truly effective it needs to be mounted much higher at my home QTH than I have it at present. The difference with the six metre version is due to the type of contact I have been looking for. When the six metre band is open I have managed to work some medium distance DX, that is most of Europe including European Russia. On four metres I am looking for fairly local inter-UK. Both six and four metres tend to exhibit both HF and VHF characteristics and so the low six metre antenna can be effective because of the HF like properties but I need to take advantage of the VHF like properties on the four metre band and at VHF height is everything.
This is a simple to make antenna and could be even constructed in a manner that it could be set up as a portable antenna on hill tops for field days etc..

Thursday 10 February 2011

4m Amplifier.


BNOS of Essex once produced a wide range of power supplies and linear amplifiers for the radio amateur, and will now still make linear amplifiers to order. Their products are also often seen on the second-hand market: look out for the LPM 70/10/100. This produces a true 100W output for 10W input, includes a receive preamplifier as well as a LED power output bargraph and SWR protection. These units are worth hunting out if you are looking for a 100W amplifier for the 4m band.
A200 amplifier
An ex-PMR linear amplifier which is ideal for the 4m band was manufactured by Pye of Cambridge. The A200 linear amplifier was designed to boost the output of AM mobile transceivers to about 50W PEP output. The E0 model (marked on the serial number plate) is designed to operate from 68 - 88 MHz and will tune to the 4m band without any modification. They are designed to operate from 13.8V and include RF VOX and RF changeover relays. The only modification which may be desirable is to provide a facility to hard-switch the amplifier to prevent the relays chattering in SSB operation. These units can be recognised by their black cast housing with a cabled input connector and chassis-mounted output connector. Don't be deceived the three core 'brown, blue, green' power cable - these amplifiers are for 13.8V DC operation. Again these units have been widely seen on surplus stands at radio rallies at bargain prices, particularly for the E0 versions.

Friday 4 February 2011

4m Halo Antenna By Gabriele Antonielli,IZ5HQB.


Photo 1
Concepts:
The halo is an omni-directional antenna with horizontal polarization. This is practically a horizontal loop, namely a half-wave folded dipole, with its endings slightly separated ,(Photo 1). The feeding is done through a system called gamma-match with the condenser obtained by inserting the central core of RG58 cable including the insulator, inside the aluminium tube that forms the gamma-match.
Assembling:
Based on a commercial model, it was reproduced using aluminium tubes from 4 and 6 mm (sold in DIY stores). The boom (H) is made with an aluminium profile 10/10 mm square section. If you are using anodized aluminium, remember that it should be grinded at all points of contact for an improved electrical conduction. 
The dimensions below can be modified during implementation to match the materials used in construction. But the important thing is to maintain the distance between E-F and B-C, (Photo 2 and Photo 3).
Photo 2
Photo 3
The E-F segment is composed of an insulating material,(PVC or wood).
The dipole is connected to the boom in the A-B side, through a screw with an insulator between the boom and the dipole,(Photo 4).
Photo 4
The gamma-match (C-G) (photo 4) is electrically connected at the point C with the dipole and isolated in G by a distance from the PVC boom. At this point it is connected the connector cable with the ground attached to the boom, while the central core is soldered of the wire (RG58) where the tuner-capacitor is done,(Photo 5).
Photo 5

Calibration:
For calibration can be used by a simple SWR meter, find the lowest point of SWR by inserting or removing the wire inside the tube (C- G gamma-match)(Photo 5). When the above dimensions has been achieved with SWR values of 1:1 from 70,100 to 70,300.
Finished Antenna


Finished Antenna
Schemitics