Jump to content


Active Members
  • Posts

  • Joined

  • Last visited

Everything posted by jobdone

  1. I wonder how many people went to the Electromagnetic Field camp in Bletchley,UK(or watched the stages online) and how many are going to the national hamfest in Newark,UK later this month?
  2. the best option is GQRX on linux. I remember running SDR# on an older version of mint - as suggested it requires mono.
  3. generally the LNB must be powered for it to work. most sat recievers put power up the coax for this (hence why you should not short this whilst powered on). the voltage varies , I can't remember exactly but it's something like 12V to make one polarization and 18V for the other. you could just pull this off and build a biquad and use this for 2.4ghz....
  4. 2% at 1090Mhz is a shaving or a slip with th knife! I doubt the RTL is 50 Ohm it's more likely to be a 75Ohm device as it's a TV receiver. J-Pole's won't have that low of an angle of radiation as although it's 3/4 wave in length it's a half wave with a J matching stub. Slim Jim's are a natural progression to a J-Pole and will lower the angle. W7PP is right about the downward tilt, this is why I suggested that a grounded quarter wave or a dipole should not work so badly..
  5. Here in the UK only FM is legal and 40 of our channels were offset from your 'blocks' , however 33 years after it became legal we are now licence free and on the verge of being allowed AM modes (AM/SSB/DSB). The SSB modes could be 12W thus allowing a licence free station on 11M more power than a licenced station (albeit a novice licence) on 10M. As CB is mainly unused now here , it allows for a bit of licence free experimenting....
  6. wideband transmitters already exist as do sdr transmitters. A good starting place is the raspberry PI. it will put out a dirty transmission from 0-250Mhz(look up PIFM) at mW (about 10 I believe) couple this with filtering to stop the harmonics and an rtl 2832u for RX and hey presto - a bit of scripting to switch the TX/RX and the addition of a mic input..... you can also use minimodem or PSK31 very easily. 10mW is alot of power into the right antenna at a good location.
  7. It surprises me that a 16 ele collinear would outperform the 8 ele , after all the planes are at quite a height and should be perfectly obtainable with a quarter wave with radials provided it's out in the clear.. I believe the 8 ele is basically just a 4 ele with 4 elements for phasing meaning a collinear made from stacked J poles with either coiled or sidewards elements should be smaller with the same amount of gain. A slim jim normally gives a reasonably low angle of radiation with very little building know-how or parts/time etc. Australian CBers make coaxial collinears for UHF. these are normally mounted on a wooden dowel with cable ties - this holds it straight.
  8. You are correct about the biquad being directional and yes it's unfit for this purpose. The biquad's reflector is wire mesh (flat) which is fencing wire over a metal frame, the mesh size is proportional to the frequency it's being used on as is the spacings in the mesh, however tinfoil over a frame could be used on all frequencies it's just not very permanant . As stated I use this is for 70CM usage , not for 27CM but scaled it wil work ok , just directional (about 70 degree beam I hear). I have a picture of the coax collinear out on a field trip with me somewhere. it is strapped to a cheap fishing pole with wire ties. the whole thing is made from RG58. I used about 2M for feeding the antenna, the ground planes are just made from pieces of 450 OHm feeder cut at just over 1/2 wave then stripped in the middle , bent into a v shape and soldered straight the the outer braiding of the coax (done twice into a cross shape which is also soldered together for strength) then waterproofed. The reflector size of a biquad I believe is about 3/4 of a wavelength square, however this size is not critical as long as it covers the biquad. What is critical is the spacing between the biquad and reflector this is altered for VSWR but is typically about 1/7 of a wavelength
  9. I have built several coaxial collinears for 70cm usage , well done for trying a 1090Mhz , this is very hard to get right due to the size needing to be perfect! I normally add a quarter wave (in air) section to the base of the antenna , then solder 4 horizontal ground planes to the outer of the coax at this point. this has 2 effects, for transmitting it decouples the coax from the antenna (as the coax will run hot) and should aid the antennas performance. I normally use RG58 for these antennas, however RG58 requires soldering and Darren's method seems great for spacing. Be careful what plastic pipe you use as some can add characteristics to your antenna! try to check whats in the plastic.... I normally leave the end of the antenna open, however I have seen the other designs with the antenna 'shorted' at the end or a quarter wave added. I am guessing that a 16ele like Darrens will have a gain of 10-12Dbd and a fairly low angle of radiation, sometimes a 4ele(with about 5Dbd) can work better in some locations due to the angle of radiation. So the pros are: omidirectional antenna with a nice bit of gain however the cons are:narrowing the angle of radiation, it is possible especially at high frequencies to TX underneath or over the antenna! and it still does not outperform my biquad with a reflector...
  10. I'm interested, is it mainly FLEX or POCSAG that you hear stateside. We get a mix over here in the UK, but it's mainly FLEX.
  11. Here is what I do to work out how far off frequency my RTL is. If you have access to an SSB transmitter and you know the frequency - use this. If not and you have some walkie talkies , then use these. Adjust your offset on your software until it matches the known frequency. I also have been known to use the raspberry PI (yes PIFM). Make sure your using the NBFM version of PIFM and try to use a known 'Legal' frequency. beware that the PI puts out some nasty harmonics without adding filtering. I have known 2 of my rtl's to be at 56ppm off and one at 32 (if I remember rightly).it is not advisable to try to tune up to a WBFM signal for obvious reasons.
  12. do a direct conversion mod on it and use it for HF.
  13. Ok for anyone looking for quick and dirty antennas a good starting place is a dipole. It has no gain (I'm talking DBd here as thats the only value that means anything - ahem dbi...). Firstly take your 300 (erm yeah close to the speed of light) divide it by the frequency in Mhz you want to RX. This gives you the wavelength in Meters. Divide this by 4 to get a quarter wave. Then multiply this by 0.95 (to allow for velocity factors of materials. make 2 lengths of this size in any conductive material - yes speaker wire is perfectly ok! then attach one piece to the inner of your coax and another to the outer of the coax. Puritans use a 1:1 balun at this point. A cheap choke (a few turns of the coax coiled) will work fine for TX - however this is just for RX you shouldn't need to worry. If the antenna is horizontal, it is horizontally polarised, Guess what happens if it's vertical?. Placing this as an inverted V is seen as vertically polarised too. The antenna legs might need to be 5% shorter for inverted V. Place this antenna as high up outside as you can get it for VHF and UHF! a low gain antenna very high will work better than a high gain antenna very low! as UHF and VHF are line of sight. The dipole should be about 60-70 Ohm. The inverted V is normally closer to 50 Ohm. For anyone wanting alot of gain on UHF always look into our old friend the biquad - it offers alot of gain and a wide angle. For VHF maybe look into a slim jim or a collinear (coax collinears are quite cheap to build). For anyone interested in a few different frequencies , dipoles can be placed together to make a 'cobweb' antenna....
  14. Yeah for this type of transmission, it's not about antenna gain - Ideally you'll need a quadrifilar helix , because it is circularly polarised and has a good 360 degree reception (yes and virtually no gain) or a turnstile antenna. This will outperform a high gain antenna due to it not being so directional.
  15. haha great, but the reason this works so well is because the guys are running it in the desert with no obstacles - a clear line of sight with the antenna up high (and no coax loss!!) and no interference. My thoughts are in the real world this would not get very far to a mobile unit - possibly across the road and a bit more? I personally believe that a biquad with a reflector (properly spaced - not like the use of a sat dish :) to another antenna of similar design would easily outdo this.
  16. Slightly off topic, but as for your 48dbi I find it amazing how antenna manufacturers never quote dbd but always dbi. Although dbi does have a formula for working it out, in the real world it means nothing! dbd however uses the reference of a dipole (a basic half wave) to compare against thus giving you an idea on real gain.
  17. jobdone


    Thanks Jason I was thinking along the lines of a jig myself. I will also be making one for 70cm so the chances are it'll be out of RG58 but then my mind wandered and with 16 half wave elements , and two quarters , a 2.4Ghz antenna would only be just over 1m long and whilst yagis and biquads have a nice bit of gain , they are also very directional....
  18. jobdone


    I was thinking about making a coaxial colinear for 2.4Ghz, has anyone had any experience in this? I think I may run into size issues i.e it will have to be millimeter perfect.I'm not really to worried about vswr because of the mW's involved but obviously a way of measuring vswr without costing a small fortune would be helpful.
  19. jobdone


    I am just trying out the new Damn Small Linux, it reminds me of the old version but mydsl is working again!!! yay!!!
  20. For years morse code has used 'shorthand' for similar reasons to text message abbreviations, it does have a place but I'd guess you need to be able to read/write proper english like what we does :)
  21. I'm not an elecrtical expert but wattage is voltage multiplied by current thus 2.49W (20.23*0.123) however I think your readings are wrong , the voltage could be on the money but I doubt your current is. How exactly are you taking your readings? i.e with the meter in series and what meter is it , i.e are you using it on a high current setting?
  22. Yeah I'd noticed VM had blocked access to thepiratebay ( I only checked because I read it on a forum ) - a quick stop at www.hidemyass.com soon sorted that out. The argument should always be that legal downloads can be obtained from P2P and are frequently done so.
  23. There are a few valid points to be thought about here, these netbooks use certain CPU's because of their low power consumption(long battery life) and quite a few are 'on-board'. There may also be a bios issue with changing CPU's - not saying it's impossible though... overclocking may also be possible if you carry a car battery and a radiator with you..
  24. Ebay UK now has quite a few of these popping up retailing average of £150+ - I don't want one that badly!
  25. Wish I could , it's further away then Cambridge
  • Create New...