Questions RE: Verticals with cap hats

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0EYJ, Feb 3, 2019.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page


    I have one of the Bosch Relays in my hand, and I just noticed that the diagram I uploaded in post #16 is WRONG!!!!

    Here is the correct footprint of the relay and wiring for the T/R switch:

    W7UUU and AK5B like this.
  2. AK5B

    AK5B Ham Member QRZ Page

    Thanks very much! Glad I asked...:eek:
  3. AJ6O

    AJ6O Ham Member QRZ Page

    The only problem I could see is if there is no power to the relay and you transmit you would be transmitting into the RX antenna which might not be made for transmitting. On my RTR-2 if there is no power to the relay it defaults to the TX antenna.
    AK5B likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    That would mean that the relay would have to be energized 98% of the time and I would have to add a transistor inverter circuit.

    Since my station is solar powered, I don't want the parasitic drain caused by the relay coil, so I guess I will have to be careful that the RCA phono jack doesn't get disconnected from the back of the IC7300. That same cable also keys the AL80b. It has not been unplugged since I built the external T/R relay...
  5. N3OX

    N3OX Ham Member QRZ Page

    Cap hats definitely help quite a bit. You can go shorter if you like and still get good performance and reasonable bandwidth.

    My field strength measurements on my 3m tall vertical with a big pyramid cap hat ( put it at about 1dB worse than the 1/4 wave wire vertical I was comparing it to.

    The coil was some house wire wound on a grooved PVC pipe. I measured its Q independently to be around 500-550 across the 40m band:

    The largest expected loss component would be the ground loss, but it didn't seem that severe with my field of 27 radials in a modest-sized backyard. I'd definitely build it again. It worked great, and given the comparison measurements and the good on-air performance I never felt like I was missing out on full-size.
    K9AXN, KE0EYJ and AK5B like this.
  6. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Really cool! Thx.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I just built a 8ft tall vertical standing on a near flat roof of a 65ft X 85ft all-steel building. Its base is at ~22ft agl, so its capacitance hat is at ~30ft agl. It has 4ea 3 ft radial wires at the top of the vertical which form the capacitance hat. The coil came from a mobile "screwdriver" antenna and is motorized (for remote tuning). The coil is located about 4ft up from the base. There is a small shunt coil across the base insulator which improves the match to 50 Ohm coax. The "ground plane" under the antenna is as near "ideal" as you can get. (almost as good as Rege's battleship ;))

    On a 500mi path (Az to Ut) on 40m, the short vertical is consistently -8db with respect to two full-sized 40m resonant antennas that I routinely use. One comparison antenna is a regular 67ft inverted V, with the apex at 55 ft agl, not much droop. The other is a 160m horizontal three-sided loop used on its fourth harmonic.

    The comparisons are done by rapidly switching (at 2Hz) between the test antenna and the regular antenna while transmitting CW at the 10W power level. I use the graphing received signal level meter at the Northern Utah WebSDR to record the difference between the two antennas. The rapid switching negates the effect of QSB during the transmit period ( usually about 20sec, so I transmit through each antenna about 10 times during the run). The -8db has been remarkably consistent when doing the comparison this way.

    There are times of the day when that path is "open", and the signals at the remote WebSDR are -60db for the big antenna and -68db for the short vertical (the resting no-signal noise level is ~120db). At other times, when that path is "poor", the big antenna might be -110db, while the short one is just barely detectable above the -120db resting noise level.

    To mechanize the rapid switching between antennas, I built an Arduino-controlled antenna switching relay which also sequences the transmitter keying so that there is no hot-switching...
  8. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Interesting, Mike. Will build bigger than 8 feet, but that's working well for you, given the size.
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    A bit more background. I started with a mobile screwdriver I bought at a ham-fest. It originally was about 6 ft high, with no capacitance hat. The coil body was about 2.5ft from the bottom. The bottom section was made from 2" od aluminum tuning. The part above the coil was a 5ft stainless-steel whip. It originally tuned 75m to 10m.

    I am converting it for use at a fixed location (another hangar roof) where it can be a maximum of 8ft tall, but it will not be subjected to anything but normal winds (in lieu of a car's slipstream). I lengthened the bottom section to 4ft (to get the coil higher). I replaced the whip with 2" aluminum tubing to act as a structural support for the cap hat. Now, it is a bit taller, and made of fat conductors.

    Now it remotely tunes from 80m to just above 20m. Adding the cap hat, raising the effective location of the coil, making it fatter, and a bit taller improved its efficiency over the original but precludes using it on 17m through 10m where it originally worked.

    This is going to be mounted on an aircraft hangar where the total height is dictated by FAA. I'll post a picture later today...
    AK5B likes this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here is what I ended up with.


    I forgot to mention that the optimum shunt coil to achieve a low Swr50 to match the RG8 requires more inductance for 80m than for 60 to 20m. I made two coils, and switch between them using a relay. The coils and relay are housed in the white weather-proof enclosure at the base of the antenna.

    This is how I initially modeled it:


    (clik to enlarge) Note the current along the vertical part and the capHat. The upper box represents the screw-driver coil. The lower circle is the 50 Ohm feed-point. The lower diamond represents the shunt coil which raises the feed-point impedance to 50 Ohms. Notice how the "counterpoise" current distributes along the wire frame grid representing the solid steel roof.
    Last edited: Feb 7, 2019
    KE0EYJ, AK5B and KC8VWM like this.

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