My only permanent fixed antennas are in the loft at only 4m agl. I have a Moonraker trap dipole for 10m,15m & 20m, it is strung north/south. Computer prediction plots on the web show that at only 4m the radiation is mostly straight up, it's a real cloud warmer on 20m with little low angle radiation, its great for working QRP around Europe but useless for real dx. It's a bit better on 15m. As you might expect there is a problem using it as its is so close to the house wiring and with 100 Watts there is considerable interference with our TV, telephone and alarm system .The switched mode power units in the house down lighting is received at S9 on 20m. but OK on 15m. I would not consider running any more power on HF with the internal antenna due to possible fire risk and damage to household equipment.
The current 2m antennas is a quad loop thrown together using plastic plumbing pipe and a length of wire, it seems to work quite well, it has no reflecters so is bi-directional. I also have a 50 MHz dipole fixed southeast which works suprisingly well and I have worked a lot of Europe using it via sporadic E. My original attempt at a 6m wire dipole is visible in the photo but even with a balun the VSWR is too high for it to be usable.
Whilst the double quad loop worked well, it is the full height of the loft which does not allow the reflectors to be fitted and additional antennas cannot be added on the same rotator. The new 4element yagi is built to a design on the DK7ZB web site and has a boom length of only 0.75m. I purchased a four element 2m yagi from the Bring & Buy stand at the RSGB Newark Rally for the sum of just £12. It had a beehive trimmer in the dipole connection, I was not expecting it to work too well and I was correct. The VSWR was unusable with or without the trimmer, but I could not buy the aluminium new for that money. The design gives element lengths for tube diameters upto 10mm, my yagi had 12mm diameter tube.I cut the tubes to the size for 10mm with the exception of the first director which was about 5mm too short. The design is for 28 ohm feed impedance so two parallel 1/4 wavelengths of 75 ohm RG 179 cable is used to match to 50ohms. The N socket needs to be grounded to the boom. Intial testing with 10 Watts forward power gave 0.5W reflected, getting better at the bottom end of the band. As tube diameter gets larger the length required tends to be shorter so I decided to trim the radiator element by 3mm each end. Reflected power reduced to 0.2 watts.Not wishing to over do it I have left it at that. In the attic I now have space to add a short 70cm antenna on the rotator pole. Result so far are very encoraging.
I constructed this antenna from a design featured on the DK7ZB web site. DK7ZB.com The boom length is just 0.6m and the claimed gain is 4dBd. The design is unusual in that it is a radiator element and director, not as you might expect a reflector. I used 10mm diameter tube for the elements and cut them 3mm longer than the design length so that I could trim them as required. The design impedance is 28 ohms which is matched to 50 ohms feeder with a quarter wave stub using two parallel 1m lengths of 75 ohm RG179 coaxial cable. The VSWR at 50.2MHz as built without any trimming is excelent, with 80watts forward power and a 100W element in the Bird throughline I cannot see the meter move on reverse power. This is a compact design and can be built very lightweight for potable operation
I had previously discounted the use of a 5 element Tonna for 6 m use due to the +3m boom length and weight on my flimsy mast. However having seen an advert for the 4 element Comet antenna with driven radiator/director (HB9CV style) I thought I would try one in an attempt to improve my 6m UKAC score. The Comet weights in at an incredible 2.1 Kg which for a 3m boom antenna is very low. I am not sure, that it would servive a gale but it is well machined and ideal for portable use. On my mast at 8m it looks far less intrusive than my 12 element 2m ZL special. After assembly, the VSWR was poor but after checking the instruction sheet I noticed that unlike the HB9CV antenna the shorting straps on the gamma match are NOT equally spaced from the boom .There is no dimension given for the shorting strap on the director but placing it at the end of the stub seems to work fine and the VSWR is now very good. Overall the antenna has noticably more gain than my two element yagi and I can now work GI4SNA without repeats. The front to back ratio for a phased array seems very good and I am quite impressed with the antenna.
The 2 element DK7ZB is now in the attic fixed beamed east, this is ideal for those flash 6m sporadic e opennings. Even in close proximity to the roof tiles the VSWR is still very good.
The boom length is just over 3m and is in two halves which are secured togther with a couple of bolts through an internal joiner tube. The design is relatively light weight and just about supportable on my lightweight Clarkes mast if the wind is calm. The claimed gain is 15dbd which is 2.5 dB more than the 9 element F9FT yagi which is just a fraction shorter. Having used this antenna for a year I can tell the gain is higher but my impression is the sidelobes are also higher which is an issue on the busy UKAC contests. The antenna has no balun as such and a simple balun may help. Sandpiper.co.uk
It is easy enought to unscrew the panel antenna from the diecast case and remove the RF and power PCBs. My first thought was to just swap the existing non standard RF connector and replace it with an N or SMA type. Unfortunately the antenna is a array of copper patches on a thin film which is sandwiched between a front and back layer of polystryrene sheet,it would be impossible to seperate them without damaging the antenna structure.The antenna plugs directly into the diplexer on the RF PCB so my solution was to cut the connector from the diplexer leaving just enough copper to solder a short semi-rigid tail. A short length of flexible microwave cable connects to a bulkhead N connector on the bottom of the case.The external interface connector hole was blank with a couple of large washers from Wilko. I used a length of 6mm studding to add a second support to the mast mounting plate. The measured Return Loss of the antenna at 3400MHz is a disappointingly low 8-9dB,at 3.5 it's 15db and at 3.6GHz it about 20dB which is obviously designed for.
Photo of my 23cm 35 element F9FT(Tonna) antenna on the mast along with the masthead preamplifier unit. TX feeders is LCF12-50(LDF4-50). RX feeders is RG58, loss of the 10m length is 6dB but who cares with 35 dB of RX preamp gain?