New ham needs some antenna insight

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ7ODD, Jun 29, 2020.

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  1. KJ7ODD

    KJ7ODD Ham Member QRZ Page

    Due to space issues I decided to try my hand building a more or less microvert for 40m as described by DL7PE, DK5AI, and OM0ET, The antenna I ended up with is about 2.6m long, center loaded, with an 8.1m counterpoise.

    I set the antenna up in the yard a few feet from the house and about 5' above ground with the counterpoise about a foot off the ground. After some some minor length adjustments the SWR was 1.2 at 100 watts/7.16MHz.

    I then mounted it next to the house and about 25' above the ground (10' above the roof). The SWR went to 3+ across the entire 40m band.

    Took the antenna back down, checked all the continuities, connections, etc. and everything appears unchanged. Put it back in the test setup and, voila, the SWR is back to 1.2 at 7.16MHz.

    The only thing I can think of that might influence this SWR change is that there is a metal gutter along the roof line a couple of feet from the antenna when it was in the test setup. When it was mounted above the roof, I tried keep the counterpoise both away from the gutter and running across it on its way to the shack. Neither made a difference.

    Could proximity to the gutter affect SWR in the manner described? It is entirely possible I am missing something else obvious to one who actually knows something about antennas. I am hopeful someone on the forum can help me sort this issue so I can get the antenna up in and on the air.

    Thanks.

    Mike
     
  2. WR2E

    WR2E Ham Member QRZ Page

    Height makes a difference.

    Can you tune it in it's final position?
     
    WQ4G, WB5YUZ and AJ5J like this.
  3. AJ5J

    AJ5J Ham Member QRZ Page

    Did you not adjust the coil for best SWR/lowest reactance when it was above the roof? Maybe you overlooked that?

    Also, if the antenna is 5 feet above ground in the first scenario, do the radials slope down to a foot above ground? How are they oriented when above the roof?

    In addition to possibly needing further loading coil adjustment there may be more ground loss when mounted close to ground and that's throwing your SWR readings off, too. If you can get hold of an analyser that would likely be of help in dialing everything in.

    Properly adjusted and loaded with a proper radial field, shortened verticals can perform very close to full-sized verticals.

    A shunt coil may be also advisable for 40m and down, too.

    73,

    Jeff
     
  4. KW4GT

    KW4GT Ham Member QRZ Page

    Proximity to ground will influence the resonant frequency, there's a good chance that you need to retune your antenna for it's final height. Also, proximity to metal can definitely have an effect and you want to keep any antenna as far away from metal as possible.

    Also, make a plot of SWR versus frequency at several points across the band (like most of us had to do before antenna analyzers were readily available) and note the frequency of lowest SWR. Your goal is to get that to be near the middle of your desired frequency range.
     
    AJ5J likes this.
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    You have overlooked that all environment around a shortened antenna
    has a very large influence on the resonant frequency, more with shortening of the antenna length.

    This is due to the large energy storage in the near-fields around the loading coil and remaining antenna portions, and it is also the reason why the bandwidth of a small antenna decreases.

    Essentially, everything metallic that is within a 1/3 wave-length or so radius of the antenna becomes part of it.

    You have to adjust the loading coil with the antenna at its final position.

    73/
    Karl-Arne
    SM0AOM
     
    AJ5J likes this.
  6. N8AFT

    N8AFT Ham Member QRZ Page

    Radial.
    Radials.
    Radials.
    Those will influence the impedance.
    You will need to adjust radiator length being above ground now.
    Are we using a 4:1 Un-Un?
    If not, why not?
     
  7. KJ7ODD

    KJ7ODD Ham Member QRZ Page

    Thank you all; this is a lot of good stuff for me to learn about and try.

    This antenna design has no radials to speak of. It uses the shield of 8.1m feedline as a counterpoise. The antenna end of the shield is connected to nothing; the other end terminates at a 1:1 balun.

    This is a link to OM3ET's project.

    If you jump to 23:05 you will see his complete antenna. Mine is similar but center loaded and longer.

    I tuned it in situ and was able to adjust it to achieve a minimum SWR of 1.4 on 7.18MHz. That was at 50 watts; at 100 watts the SWR was 1.5.

    Will adjusting the loading coil improve that SWR? The original coil was sized for 28.4 microF. The same on-line calculator now gives the required inductance as 25.2 microF due to the increased length and slightly altered position relative to the feed end. That reduces the coil by 7 turns.
     
  8. KC8QVO

    KC8QVO Ham Member QRZ Page

    Interesting antenna.

    I made a vertical around 2007 or 2008 out of a 16 foot tall crappie fishing pole (telescopic pole sections with a single eyelet on the tip section), plastic coat hangers, and coffee can lids that worked absolutely wonderfully.

    The coil was made out of the coat hangers and coffee can lids on about a 5" diameter form. I cut the coat hanger material in to rods about 5" long then used a saw to cut grooves in them every 1/4" or so. Those grooves became the slots for the coil wire - around 20 gauge insulated solid wire.

    I made a tap for the coil out of copper coated staples - the kind you get from on big cardboard boxes. I cut them and bent them in to a clip that was easy to clip on to the coil wire (with the insulation stripped off where the tap was to go).

    The coffee can lids were cut in circles that matched the diameter of the coil. Then in the middle I cut holes in them. With the crappie pole being telescoping sections I cut the coffee can lid on the top of the coil so that it would rest at the top of one of the crappie pole sections while the bottom coffee can lid was sized to slip over the bigger section below.

    The antenna was intended to be a portable vertical antenna. The length being 16 feet meant the full length of the antenna was too long for 17-10 without shortening it, but 20 and 40 worked beautifully on the full size antenna.

    I agree with what others have stated - the shorter the antenna and the more loading you use to resonate it on a particular band the narrower the bandwidth.

    Also keep in mind that a ground mounted vertical (I know yours is likely going to be elevated) will have a very low impedance at resonance - perhaps 25 ohms or so, NOT 50 ohms. When you get in to elevated radials - the length of the radials and the angle of them with respect to the vertical radiator will adjust the feed point impedance. Depending on what your feed point impedance is it might be beneficial to use a 2:1 unun at the feed point to bring the impedance of the feed point closer to that of the 50 ohm coax.

    If you only use 1 radial what you essentially have is a dipole where the vertical is one side and the radial is the other side. This means that your antenna will be most sensitive/directive towards the direction you run the radial. When you introduce more radials then you start to balance out the pattern more. If you have 2 radials 180deg apart then your antenna will be omnidirectional. As that separation angle decreases the direction of most sensitivity will be mostly in the direction of the smaller angle.
     
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  9. K8JD

    K8JD Ham Member QRZ Page

    You need to chart the SWR from below the band to above to see where the low SWR point has shifted to and tune the antenna to compensate for that.
    The short, loaded antennas are not efficient away from the low SWR point and my big, full size wires work over a large portion of the bands.
     
  10. KJ7ODD

    KJ7ODD Ham Member QRZ Page

    I checked the SWR across 40m and the lowest was 1.4 at 7.18MHz. When you say "tune the antenna to compensate for that" what do you mean? I'd like to get the SWR lower on that frequency.
     

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