Callsign
ad: rfparts
Page 2 of 4 FirstFirst 1 2 3 4 LastLast
Results 11 to 20 of 31

Thread: First attempt at a dipole

ad: L-HROutlet
ad: l-assoc
ad: l-WarrenG
ad: l-rl
ad: l-tentec
ad: l-gcopper
ad: l-innov
ad: l-Waters
ad: l-sarc
  1. #11

    Default

    Thanks for the compliment. I tend to think things through too much sometimes, building it in my head so many times before I actually pick up a piece of wire.
    Yes-- an analyzer would be great. I'm in the middle of Nowhere, MT so I'll have to add it to my wish list. But if I get a tuner first, might that also help a lot if I'm far off? I had a tuner on my wish list but closer to the top...

  2. #12
    Join Date
    Feb 2010
    Location
    Dallas / Fort Worth
    Posts
    3,550

    Default

    Before you start cutting and trimming, create a spreadsheet and record / graph the SWR versus frequency for each band to get an idea of where you are at length-wise. The lowest attainable SWR reading will vary according to whether or not the dipole is set up as a flat-top or an inverted V -- inverted Vs tend to have a feedpoint impedance closer to 50 ohms. The height may also be an issue at 40m, but you should be able to get it all to work.

    The only issue you might run across is if you have a metal vent on the peak of the roof or if you have a cast iron vent pipe nearby. Some modern roofs also have little aluminum clips locking the decking together, but they are spaced apart pretty well.

  3. #13

    Default

    Nope, no ridge vent. No gable vents, either. Its all soffit-vented around the perimeter. The galvanized exhaust vent from the kitchen stove runs up past where the antennas will pass, though. Maybe 8-12 inches away. Any other vents are over 8-10 feet away. No clips on the plywood, either.

    Good idea re: the spreadsheet!

  4. #14

    Default

    What you are likely to find out using only an SWR meter and your legal frequencies is that you are off frequency but may not know which way, so take a series of readings starting at one end of band and working down - you can use low power AM and voice ID if that's easier for you. You numbers will give you a trend to tell you if your wire is too long or too short, but not how much. If the band edge is 2:1 on 40, I'd make a 6" adjustment then check again, maybe a foot if it's 3:1, then re-measure to see if you are closer.

    Since each band interacts, you should first tune 40, then 20 then 10, then go back and check - since each band interacts. I always attach the end insulators with split bolts or other temporary fastners while I do all this, then finalize after I get the match working to my satisfaction.

    If you are serious about building antennas, you are going to need a good SWR measuring device anyway. A lot of us have the MFJ's and they work well, another new option seems to be from Ten-Tec, the YouKits FG-01 which even has a color screen and looks like it might be a good choice for HF. There are a couple other options from other vendors.

    I feel the ability to go outside amateur band limits for tuning - which can be handy when doing utility listening for example - is a good arguement for the analyizer over the ordinary SWR meter.
    This Space Intentionally Left Blank

  5. #15
    Join Date
    Feb 2010
    Location
    Dallas / Fort Worth
    Posts
    3,550

    Default

    Yep, what NA0AA said.

    For years I used an SWR meter, and looked at analyzers as an expensive toy for folks who couldn't use an SWR meter. However, as I got into modeling antennas and seeing what the SWR looked like plotted over a large range of frequencies and how angles and lengths affected the readings, I decided an analyzer would be nice. I chose the AA-54, as it had a graph and worked very similar to the modelling software as far as the SWR display. It has been great for setting up taps and working out counterpoise lengths on a Buddistick, and when we needed to measure some stub filters, it was definitely very useful. Something to consider once you catch the antenna building bug.

  6. #16
    Join Date
    Jan 2000
    Location
    Commerce MI (Detroit area)
    Posts
    8,060

    Default tuning a dipole or other antenna

    I agree with making a plot across the whole band with as little power as the meter in your rig will allow a reading.
    My TS440 SWR meter reading can be done with about 1 watt. Start at the low end of the band, take a reading and move up 25k and take another until you have the band plotted.
    If you can get each reading in a second or so of transmission it will never be heard by anyone.
    I usually use 8 x 11" sheets of 1/4 inch graph paper and plot the freq across the bottom in 25 kHz increments and SWR readings going up the left side of the graph. Make a page for each band. This will show you a "bell" curve and tell if the dipole is too long or short. Trim and replot if you are not in the right part of the band. Save the charts for future antenna modifications.

    Have fun.
    Last edited by K8JD; 09-13-2012 at 10:19 PM.
    73.....JD, FISTS #3853,cc 455,SKCC # 1395,tribune #12,
    Official US Taxpayer

  7. #17

    Default

    Hm. I like that idea of plotting in 25kHz steps. Lots of work but it sounds like it will yield a better picture. I strung the whole thing up this morning while the attic was cool enough. Starting to play with it now. The first run through showed SWR a little too high at the lower ends of each band and little higher at the high ends. That was before i read the suggestion about plotting the whole band. Back to work!

  8. #18

    Default

    OK. Its up. SWR looks high and just about every band...
    10m 28.000 28.100 28.125 28.150 28.175 28.200 28.225 28.250 28.275 28.300 28.325 28.350 28.375 28.400 28.425 28.450 28.475 28.500
    SWR 3.000 3.000 3.000 3.000 3.000 3.000 3.200 3.200 3.200 3.200 3.200 3.200 3.200 3.200 3.200 3.200 3.200

    20m 14.025 14.050 14.075 14.100 14.125 14.150 14.175 14.200 14.225 14.250 14.275 14.300 14.325 14.350
    SWR 1.500 1.500 1.600 1.750 1.750 1.750 2.000 2.200 2.300 2.300 2.500 2.600 2.800 3.000

    40m 7.025 7.050 7.075 7.100 7.125 7.150 7.175 7.200 7.225 7.250 7.275 7.300
    SWR 3.500 3.500 3.500 4.000 4.500 5.000 5+ 5+ 5+ 5+ 5+ 5+
    When its more then 3:1 I can only guess since its the built-in SWR meter. I'll climb back up and start fiddling with the lengths (after company leaves next week). Starting with the 10m segment. Right now the ends are fed through the plastic insulators and wrapped back around themselves 2 or 3 or 4 turns. If I want to shorten them, should I wrap them more or just fold them back along the line, sort of an elongated "U"? In other words, slip the line into the insulator and fold it back on itself straight along its length and secure it with a zip tie or tape... I don't want to physically cut until I know I'm on the right track.

  9. #19
    Join Date
    Oct 2008
    Location
    Louisiana Gulf Coast
    Posts
    1,019

    Default

    All looks well done!

    And you should be able to work 15 meters off the 40 m elements. You will need a tuner to touch up the 15 m band, but it will work nicely.

    For the 718 (I have two, one for portable/field day, one up at the club station) let me highly recommend the LDG IT-100. They work and play well together.

    "just fold them back along the line, sort of an elongated "U"?" Yes, exactly.

    Small cable clamps on the ends will let you quickly and easily take in, or let out wire for tuning.

    Looks like all of the elements are too long. Start tuning with the longest, 40 m elements. Your 20 meter elements are not too far off, so shorten them just a little, a few inches each end.

    A few years ago an Autek RF-1 Analyzer showed up in my Christmas stocking. Works well, and more than good enough for what you are doing. Small, slips in my shirt pocket, very handy for Field Day, etc. And uses a common 9v battery.

    73,
    Paul - AE5JU
    Last edited by AE5JU; 09-17-2012 at 01:56 PM.

  10. #20
    Join Date
    Feb 2010
    Location
    Dallas / Fort Worth
    Posts
    3,550

    Default

    As Paul mentioned, it looks like they are all a bit long. As you trim, you should start seeing the SWR going back up on the lower frequencies -- the lowest point you see above this will be the resonant frequency. If you want to use the antenna for the whole band, center it within the band -- if you want to work only the CW or phone sections, center it within this part of the band.

Page 2 of 4 FirstFirst 1 2 3 4 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •