I use a mixture of commercial ham radio antennas and space saving home brew indoor wire dipoles for the HF bands. These include an attic Inverted V dipole for 20m and an HF Ground Plane for 10m, both of which are ideal for limited space installation or for a property with restrictive covenants on the use of outdoor antennas.
In my case, I have quite a small garden with no mature trees to anchor antenna wires on, so I have been forced down the path of using indoor HF antenna designs.
I've spent a while optimising them and in some cases they offer good performance, with only the shortened 40m dipole being rather disappointing for DX work. One of the downsides of indoor dipole antennas for amateur radio is noise pick up which can be a problem and the noise floor on my 20m Inverted V dipole is high at around S3.
I now use a Diamond X-400 collinear antenna for 2m and 70cm, having recently upgraded from a Diamond V2000 tri-band 2m, 6m and 70cm antenna. The X-400 is a centre loaded 3 x 5/8 wave on 2m and a 7 x 5/8 wave on 70cm. The gain figures are 7.9dBi on 2m and 11.7dBi on 70cm. The fibreglass FRP outer shell is 4.05 metres long (13 feet). I have the Diamond X-400 mounted at around 40 feet on a 10 foot 2 inch pole on the chimney.
The Diamond X-400 is an good antenna, giving between 1 and 2 S points improvement over the V-2000 on both bands extending my coverage to around 70 miles in most directions on 2 metres. Click here for predicted coverage plots from my QTH, by Radio Mobile software.
I am currently feeding the Diamond X-400 with 13 metres of RG213 coaxial cable, since I had so many problems with the H-100 feeder I used on the Diamond V-2000 causing a poor SWR. The SWR problems are now fixed and the X-400 SWR readings are a respectable 1.2:1 (2m) and 1.3:1 (70cm).
The only problem with the Diamond X-400 is it bends and flexes significantly in high winds and this leads to rapid signal variation on 70cm (UHF). In recent gale force winds, received signals on 70cm from a local repeater varied from S9 down to S4 due to radiating element movement. The problem is less noticeable on 2 metres. I personally doubt whether this collinear antenna will survive the great British weather given the sustained high winds and storms we're getting battered with this winter (2006), but I can only hope!
My previous antenna, the Diamond V2000 3 band vertical was at best adequate on 6m. Click here to see my old installation. The V2000 wasn't really designed for the SSB end of 6 metres. To get a good SWR, I left the tuning radial a little longer than needed for better SWR on the SSB end of the band when I installed it. The TS2000 internal ATU takes care of the 1.8:1 SWR at 50.150MHz reducing to to 1.1:1. It still isn't an ideal antenna for SSB use as its vertically polarised and directional (depending on the orientation of the 6m radial element). However in the F2 season of 2001, I worked many East Coast and Mid West USA stations and some Canadians. I have also had contacts with Greenland and Iceland.
Sporadic E brought many European countries too up to 3000 miles or so distant with excellent reports. Highlights were XW0X in Laos (near Burma) and South Africa by F2 propagation. To be honest this aerial really isn't that spectacular on 6m and it's very directional too as it only has one tuned counterpoise radial and that needs to face in the direction of best performance. It seems that the aerial has a null 180 degrees from the direction of the counterpoise, even though it is supposed to be omni directional! Tests show my 6m coverage was very limited. Interestingly, a loft mounted 6m dipole works just as well as the V2000!
I found an excellent ARRL article with great tips on installing and tuning attic mounted aerials for the HF bands. It covers all aspects of shoe-horning in dipoles and loops into tight roof spaces and is ideal for Amateurs who have restricted covenants on their property or want hidden antenna systems. It doesn't cover the issues of potential fire-risk and radiation hazard due to high RF field strengths inside the house and these should be separately investigated.
I also advise that all indoor antennas be fitted with a ferrite ring 1:1 balun to reduce feeder radiation. My experience shows that this can really help reduce TVI and local interference. You can read limited space antennas on the ARRL website for more tips and indoor antenna ideas.
I currently use an experimental home made Vertical Quarter Wave Ground-plane antenna for 10 metres mounted inside the attic of my house. This aerial was installed in September 2003. Initial tests have yielded remarkable signal reports. On January 4th, 2004, K2KV in New Jersey informed me that I'd managed the strongest signal he'd heard from Europe all morning at 5/9 even with mediocre band conditions. Considering other stations were running beams, this is very encouraging. This antenna shows real improvements on the indoor Inverted V that I used before for 10m, possibly due to its lower angle of radiation.
The diagram above shows the basic 10 metre ground plane design concept - click on the above image to enlarge it. The main vertical radiator is just a quarter wave wire made from 79 strand speaker cable. The antenna uses a full-length quarter wave counterpoise wire which I have facing WNW to favour the USA. There is also a 1.2m (shortened) counterpoise wire facing south.
The aim, by placing the radials in this pattern, was to get the antenna to radiate from north west through south west to south. Using thick gauge wire and having the other radial/s half length gives an excellent 50 ohm match (1:1 SWR at 28.45MHz) and superb bandwidth right across from 28.0MHz to 29.7MHz limits where the SWR is still better than 1.5:1. I feed the antenna with a short section of RG58 then use a 'N' type plug/socket combination to convert to RG213/U cable. The RG58 is used to wind the balun on a 40mm ferrite ring (see picture opposite). This helps reduce TVI and noise pick up from the house wiring through the coaxial cable outer screen. More turns on the ring are advised if RFI and TVI is still observed.
Unfortunately, in my case the antenna is adversely affected by the loft water tanks which are only a few feet away to its east; which means that my performance on 10 metres is poor towards Asia.
Due to the worsening conditions and to allow room for the new 40m antenna, I have reverted to a flat dipole antenna facing south east - north west in the attic. After careful tuning I have managed to get the SWR better than 1.15:1 across the whole band. This antenna is facing NW-SE in the hopes that it will work for US contacts when 28MHz eventually closes.
In early May 2004, I installed another dipole, carefully positioned to avoid interaction with the other inverted V and dipoles in the attic. This antenna is mounted NW-SE in the opposite side of the attic to the 15m dipole with a separation of at about 15 feet. Initial reports seem good, however band conditions have been very poor since the antenna was installed. Best DX so far is KW7Y Seattle, WA, USA which is quite an achievement given the band conditions and terrible static interference from storm activity across the UK and near continent.
I use a separate Inverted V dipole for 20m also attic mounted and is made from 79 strand speaker cable to get a bit better bandwidth (due to the thicker wire). This antenna has worked ZL and VK quite a few times and faces NE-SW. Interestingly, it does show some directional characteristics, favouring south east Asia, which is strange as antenna theory depicts that the aerial is too low off the ground to exhibit much in the way of directivity.
Following an extensive period of installation and testing, the new GM4JMU shortened dipole is now operational! This is based on the web article from GM4JMU - 40m short dipole. My version actually uses 29 close wound turns of 24 strand insulated wire on the 40mm former as a loading coil and the antenna is set up in the loft as an Inverted V.
The 29 turn loading coils are easier to make, work as well and more rigid. The antenna is about 9.9m long. If you choose to wind the coils at 40 turns each, then they must be spaced over the entire length of the 160mm long 40mm diameter former and not close wound otherwise resonance occurs too low at 6.5MHz.
If you have a GDO, it would be good to dip both coils onto the same frequency. I haven't got a GDO so it's pretty much guesswork, but I spent time accurately winding the coils to be visually identical and this is much easier to do if the turns are close wound (hence my using 29 turns close wound). Then paint the ends of the windings with emulsion paint (if using indoors) to stop the windings moving. Even a less than a millimetre movement in the winding's structure can quite noticeably change the tuning. Even moving your hand six inches from the coils affects the frequency of resonance quite noticeably. You should also expect the large temperature variations in the loft to alter the shortened dipole's tuning which can make setup very fiddly.
Short Dipole SWR
7.00MHz - 1.2:1
7.05MHz - 1.1:1
7.10MHz - 1.2:1