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..

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