Analysis of 2m/440 rollup antennas

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KX4O, Nov 3, 2018.

ad: L-HROutlet
ad: l-rl
ad: Left-2
ad: Left-3
ad: Subscribe
ad: L-MFJ
ad: MessiPaoloni-1
  1. KX4O

    KX4O Ham Member QRZ Page

    Has anyone here performed actual laboratory analysis of the various rollup 2m/440 antennas available from N9TAX, Ed Fong, Nelson Antennas, or others? If so, what did you find? I'd like to compare your results with mine in an upcoming article.
  2. KV4JW

    KV4JW Ham Member QRZ Page

    It really doesn't apply to what you're looking for, BUT, for what it's worth, I built a 2M "roll up" out of 450 ohm ladder line and it works well. Tuned down to about 1.3 SWR and didn't notice any common mode on the coax feed with a ferrite bead snapped on it.
    N0TZU and KX4O like this.
  3. KD6RF

    KD6RF XML Subscriber QRZ Page

    I didn't perform gain pattern tests (there's no reason to doubt what the models show, which is a decent few dB low gain somewhat bumpy dipole-like pattern).

    The current measurement tests I did indicated that the ferrite choke provided on the N9TAX unit I have is necessary to keep undesirable common-mode current flow off of the antenna feed line. Common mode current on the feedline makes the feedline an undesirable part of the antenna, and skews the pattern which almost always lowers the gain in the horizon direction that we want.

    Without the choke, feedline common mode current was only a few dB down from that on the radiator. With the choke, it was at least 10 dB down under any conditions of feedline length that I tried.
    Last edited: Nov 4, 2018
    N0TZU and KX4O like this.
  4. KX4O

    KX4O Ham Member QRZ Page

    Thank you N4JNW and KD6RF. These are both valuable observations that align with what I am seeing. I've measured the gain pattern of a few makes/models so far and have another couple on order to add to the mix. One clear consensus from the three of us is the absolute requirement for some form of mitigation of feed-line currents... something stunningly missing from some prominent vendors.
    So far the measured gain patterns more or less match those from simulation including the times where feed-line currents exist. That has been a comforting finding.
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Does rolling the antenna up on a toilet paper roll in the lavatory count ?

    I have heard users using the N9TAX antenna, They have a good signal.

    Just saying that you use a jpole type antenna on the air will make your signal drop 3dB.

    Have Fun.
    NH7RO likes this.
  6. NH7RO

    NH7RO Ham Member QRZ Page

    If you roll up your J-pole before tossing it will fit in the trash can better.
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I can actually see the value in such a design, as it occupies almost zero horizontal space and can be taped to a window, or hung from the ceiling with a thumbtack, etc.

    Better than a "rubber ducky." :)

    For a "home station," I certainly wouldn't use one.:p
    N0TZU likes this.
  8. KX4O

    KX4O Ham Member QRZ Page

    Drop 3 dB from what?
  9. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    They work one heck of a lot better than a rubber duck antenna - of course most antennas do, LOL! They weigh very little and are great to have along for the back country or for an ARES deployment, etc. I made it years ago when you could still get 300 Ohm twin lead.
  10. KM6LYW

    KM6LYW Ham Member QRZ Page

    N9TAX is my primary base antenna, extended above roof by 9' fiberglass flyrod. Radiation pattern is notoriously flat with a folded jpole so make sure it's plumb. Over 100miles VHF simplex at 5-50 watts (5 being noisy). With oak trees in the background, it's almost impossible to see from the street. I have a 300ohm backpacker version as well, VHF only -- hoisted up trees in the woods, never fails to impress.

    VHF SWR readings are 1.03 at mid point at 5watts with ~20' of LMR240. no more than 1.15 overall. 49watts driven forward, one watt reflected.

    Freq SWR (watts)
    144.390 1.10 (4.9W)
    145.700 1.03 (4.1W)
    146.520 1.10 (4.9W)
    147.360 1.15 (4.8W)

    443.005 1.53 (3.3W)
    446.225 1.30 (5W)
    448.925 1.02 (5W)
    449.475 1.01 (5W)
    Last edited: Nov 6, 2018
    KX4O likes this.

Share This Page