Douglas-fir foilage muddles 70 cm signal

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KI7MBR, Jul 19, 2021 at 11:39 PM.

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  1. KI7MBR

    KI7MBR Ham Member QRZ Page

    Greetings all,

    If useful, some brief info to add to another thread now closed "Do trees block ham radio signals?"

    First trial. At a recent local net, I set up an Ed Fong DBJ-2 (portable roll up uhf/vhf j-pole) suspended from a branch about 12 feet above the ground on a bushy second growth Douglas-fir tree. Its shade was most welcome on a early July summer's eve here in Washington. The radio; 5W Icom T70. Testing on 2m, I connected with a ham from a repeater 35 miles distant. Not a super good signal, but intelligible. Thinking all was good. I waited for the net to open, first on 70cm from a repeater about 6 miles distant and then 2m simplex.

    Results. On 70 cm I could hear everyone quite clearly, nearly 5/9. But,... on Tx, whereas my signal was strong, and voice recognizable, the signal was too muddled to be decipherable. On 2m simplex all was loud and clear, eliciting comments as to "what kind of antenna are you using, the speaker nearly fell off the wall."

    Those results led to the discovery of term "multipathing." The randomly bushy Douglas-fir foliage apparently dispersed the signal along a number of paths each arriving at the repeater at slightly different times. Not the same as multiplexing, I was corrected on that. At this time of day and year, foliage in our local conifers would not be flush with water as say compared to spring time. But still and yet enough to fuzzify the signal.

    Second trial. A week later, suspended the DBJ-2 from paracord stretched between house and a lower branch on a 30 inch diameter western red cedar. Top of antenna ~10 feet above ground. Open sky above. Towards the repeater, the nearest conifer was about 200 m distant. Same radio at 5W.

    Results. For the repeater 5/9 both ways. Loud and clear. For simplex, same as before as in, just what are you using?

    Morals of the story for this novice.
    • avoid being directly under bushy vegetation, especially conifer, when attempting 70 cm
    • the Ed Fong DBJ-2 is a handy and powerful kit for the field
    • and, the alpine butterfly knot greatly facilitated attaching the antenna approx. amidships on the paracord in the second try.

    End of report. 73 to all, KI7MBR
     
    KO4ESA likes this.
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    Try horizontal polarization. Foliage has a predominantly vertical absorption profile. FM broadcasters figured this out in the early 1960s
     
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  3. KL7AJ

    KL7AJ Ham Member QRZ Page

    Here ye go
     

    Attached Files:

    K4EEO, W2EV and KO4ESA like this.
  4. KE8OKM

    KE8OKM Ham Member QRZ Page

    Good info! I am a VHF SOTA activator and use a N9TAX Rollup-Jpole usually hung from a tree 10-15’ off ground and also a Handheld Yagi.

    The Rollup has helped me activate mostly on 2m, about 120 SOTA peaks. The remaining 30 or so were with the Yagi. I have found that foliage, at least on 2m has a negligible effect. I do not have enough data points with very few 70cm QSOs.

    Longest QSO Rollup = 135 miles.
    Longest QSO Yagi = 234 miles

    both with FT60R at 5w.

    Erik
    KE8OKM
     
  5. K1JNT

    K1JNT Ham Member QRZ Page

    Back in the day when TV service guys installed antennas, they knew this stuff...keep it away from the trees...and for some reason conifers were worse.
     
  6. W4ZNG

    W4ZNG XML Subscriber QRZ Page

    Some time ago I found out a similar thing while using FRS radios in a salty island pine forest. 65cm/4 = 6.4", which is approximately the length of the pine needles. This of course makes those pine needles very good RF absorbers. The effect was uncanny. My brother would step just a few yards into the woods and all comms would cease, then he could come out and walk a hundred yards along the tree line with no comms effect.

    I wonder how long those Douglas fir needles are? 70cm/4?
     

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