VHF+ receive antenna ideas?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K3RW, Sep 20, 2019.

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  1. K3RW

    K3RW Ham Member QRZ Page

    Been dabbling in contests and also chasing distant SOTA activations on VHF and above. Sometimes they hear me better than I hear them. With the mountains all around, beam headings are frequently meaningless. I'm pointing sometimes at Mt Hood to get a bounce into the coast range, etc. And for a Seattle station, its a long haul for low hanging yagis pointed that way. From my QTH I need more receive power, or go up in the mountains.

    If I was on HF, I'd consider a beverage antenna if I could point it that direction of interest. Or a skyloop on a single band to catch signals if they tended to originate around me.

    But on VHF and above, seems that horizontal stacking of loops (to get a more omni pattern for SSB), and phasing verticals for FM (I've never actually seen this), seem to be my only options. I've not seen much else besides a rhombic suggested.

    Any others I'm overlooking?
     
  2. W5LZ

    W5LZ Ham Member QRZ Page

    Maybe the most practical thing would be to just get your antennas (of whatever kind) higher. For some, that 'higher' thingy is more impractical than other things (me, for instance). But, it's the single one thingy that will improve received signals the most (on any band). The best that you can do is just that, the best you can do in your particular circumstances, which is different for everyone. Good luck.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Since VHF+ propagation is almost entirely tropospheric and not ionospheric, when a single antenna is tuned to match its transmission line impedance, it is almost always reciprocal - it will behave the same way for transmitting and receiving; I've been on VHF-UHF for 50+ years, including running some pretty big stations (1500W and stacked beams on towers and stuff) and have never heard of anyone using a separate receiving antenna.

    What I would "guess" you're experiencing if others hear you better than you hear them is a situation where the others are running lower transmitter power than you are (really common with SOTA stations, since many of those are battery powered and running 5W or so) or that you may have a higher noise level than they do. Of course, it could also be your receiver(s) lacks sensitivity! On HF, "sensitivity" isn't much of an issue since atmospheric noise almost always limits usable sensitivity anyway, but on VHF-UHF+ it becomes a pretty big factor and I've measured many VHF-UHF rigs to find their sensitivities vary a lot; some are really good, having a NF below atmospheric noise and some are quite poor, having NF above atmospheric noise.

    So, your situation could be caused by lots of things; but improvements are generally made by making antennas bigger and higher and using them for both TX/RX and not separate ones for RX.

    "Horizontal stacking of loops" doesn't make much sense, although maybe I don't understand what you meant by that. VHF-UHF SSB/CW activity is all horizontally polarized, so to increase gain we stack loops one above another and some use as many as eight of them stacked, which can easily occupy four wavelengths of vertical space; activity on FM is all vertically polarized, and to increase the gain of a vertical (omni) we usually stack the elements vertically to make a collinear antenna, some of which can also be as many as eight stacked and phased elements (on 70cm) or maybe 3-4 stacked, phased elements (on 2m); but we don't normally use phased vertical arrays with horizontal spacing between them like some do on HF.

    HF guys do that because stacking vertical radiators one above another becomes very tall (for an omni); and they do it also to phase lag or lead verticals to create directional systems and "electrically steer" the antenna in different directions. On VHF-UHF, no need to do that when beam antennas are small and light and easy to install (compared with beams for HF).
     
    K3RW likes this.
  4. K3RW

    K3RW Ham Member QRZ Page

    The loops would be those horizontal-omni type things, like the M2 halos or PAR Antenna. I've had those stacked (or phased) before and got decent reception compared to just one loop. Of course the gain is essentially 0dB, or even slightly negative, but that was a decent tradeoff for signals coming in from all sides. So for SSB/CW/digi, I was okay with that when I had them. I suppose I could always run 2, 4, 8, etc., on appropriate power dividers for more omni receive gain. Or stack yagis on a frame for more directional gain--or just get bigger ones.

    [​IMG]

    For FM though, I'll run yagis for the quieter guys, but I do relatively okay with a small beam and a collinear. But I'd like to up the received signal, and wondered if I could potentially phase two vertical 2m collinears, though I suppose that's one solution to getting better FM vertically polarized gain.

    Good point on the reciprocal part. At times this QTH is noisier than I'd like, and the SOTA guys are frequently lower power on handhelds or portable rigs like the FT-817s. I can run higher power to them without any difficulty. Though I've noticed that based on their locations, there can be significant multipath problems.

    If I just wanted bigger gain in one direction, I'd stack a yagi set and point and shoot. But in sprints and some contests, the signals come from all over. No point in losing half or more with the yagi pointed south for that moment. And at least here next to Portland I've run about 50/50 FM to SSB. In my log, FM doesn't get many grids (4 if I'm lucky), but it gets easy 60-70% of the total QSOs. SSB gets maybe 7 grids but many fewer contacts, maybe 1 in each grid.

    I'd like to have a yagi for point and shoot and a stacked set for receive if the signals are close-in and all over. But for FM, I don't see a decent alternative. I have 2 FM beams, but aside from a single collinear, I guess phasing one (I may be misusing the term) for an extra 3dB gain is possible, like what the CBers try to do.

    I am definitely looking into LNAs too.
     
  5. K3RW

    K3RW Ham Member QRZ Page

    How do I test the sensitivity of a given receiver? I am curious which of my rigs has a better 2m, 6m, etc. sensitivity. If we had beacons nearby that would help. I haven't seen sensitivity figures of the IC-7100 compared to my monoband TM-271, but I suspect the ICOM is more sensitive on 2m FM.
     
  6. W4OP

    W4OP Ham Member QRZ Page

    Inquire from the rig manufacturer what the noise figure is of their rigs on VHF/UHF.
    Also, calculate the loss in your feedlines- feedline loss mathematically adds directly to the NF number.
    Then and only then can you decide whether an LNA will help your system noise figure. An LNA in the shack rarely helps. The most advantage comes from mounting the LNA at the antenna. It all comes down to math- no guessing required.

    Dale W4OP
     
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  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I use a signal generator. I don't have any easy way to measure noise figure, but do have a couple of real lab-type signal generators that are well shielded enough to measure down to -140dBm or so, so it's pretty easy to compare receivers just by reading and recording the minimum detected signal in a given bandwidth, or maybe 12 dB SINAD (which will be a higher signal strength number than MDS), and then just comparing them.

    The "sensitivity figures" published may not be terribly meaningful, but they're better than nothing. One reason I believe they're not very meaningful is I measure pretty big differences between rigs of exactly the same model; as such, the published figures may be based on the design, or measurement of one test sample or something. Since usable sensitivity is also related to receiver bandwidth, two different model radios may measure quite differently just based on filtering.

    And then, there's "truly usable sensitivity" vs. "sensitivity measured in a screen room in a closed system with a signal generator." Truly usable sensitivity would dictate having the rig connected to a real antenna exposed to hundreds of sources of RF across a wide spectrum, as it's easy for "wideband receivers" to become desensitized by bombardment of interfering signals outside the band.
     
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  8. W1VT

    W1VT Ham Member QRZ Page

    The proper test signal generator for most hams is a surplus HP 8640. Lots of them were made so they can be found on the surplus market. A cheap signal generator rarely has the shielding needed for receiver measurements.
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I have an 8640B and also a Boonton 102C; they both support measurements in the submicrovolt range just fine.

    But again, "usable sensitivity" (antenna connected) and "measured sensitivity in a closed system" can be two very different things, especially with the very wideband VHF-UHF transceivers so common today.
     
  10. WA9SVD

    WA9SVD Ham Member QRZ Page


    Since you are working portable for SOTA, a Yagi isn't out of the question. Using the "Armstrong" method (i.e., manually turning the antenna) may be an option.
     

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