2 Meter foxhunt antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KK4JW, Mar 26, 2021.

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  1. KK4JW

    KK4JW Ham Member QRZ Page

    I'm interested in building a foxhunt antenna for 2 meters, and am tossing around ideas. I'd like some advice and input of the antenna gurus here, please.

    I see a lot of these "tape measure" yagi designs floating around, but I don't want to do that. I just don't like the idea of a cut up tape measure flipping around, and plus, it doesn't look very.. Cool? Plus, EVERYONE seems to use those, and I'm not everyone. I want an edge. I want something that PERFORMS. Due to that, my questions mostly regard the actual design and measurements.

    - Will adding additional directors increase the RECEIVE-ability of the antenna? Or do directors merely focus the transmit radiation pattern more sharply?

    - If I want sheer receive gain, would adding multiple reflectors help there?

    I understand while not required, a small adjustable attenuator would be handy to have. Anyone have a link to a design for something I can whip up with a few parts from the junk drawer?

    Lastly - and I know this is a long shot - but if anyone out there is bored and would graciously care to model something on EZNEC for me, I'd be forever grateful.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Attenuators should not be needed until you're quite close to the transmitter (or its antenna), and if using typical ham gear, it's often not well shielded enough for an outboard attenuator to work much. A plastic-cased hand-held can receive signals right through its case, even with no antenna connected -- so when that happens, an outboard attenuator is senseless.

    But...and this is a big one...you really need to have a receiver (more likely a transceiver) that has an S-meter that really works on FM signals. Most are terrible and measure limiter current, so the real difference between an indicated "S2" and an indicated "pinning the meter" may only be 10 dB -- when it really should be >60 dB. This is where an SDR would be really helpful. The only transceiver I have whose S-meter actually functions properly with FM signals is my old FT-736R. If a signal changes strength by 3 dB, the meter shows that; if it's 30 dB, the meter shows that, also: And to "pin" the meter requires an amazingly strong signal. A few mountaintop LOS repeaters can pin it, but they'd have to be truly LOS and pretty close. Plus, it has a real RF Gain control that works on all modes including FM. And it's well shielded, so an external attenuator actually works.:p

    In any case, I don't like "tape measure" yagis, either. A fully assembled, full-sized 3L or 4L 2m yagi easily fits in my trunk or back seat, so I've never had any need to make one more portable than that.

    Lots of small yagis or quads or whatever can to this job just fine, but the real important element is the receiver and having an accurate way to view a signal peak, even when the signal is strong; with a strong signal, you cannot "hear" a signal peak, you need a visual indication.
     
  3. AC0GV

    AC0GV Ham Member QRZ Page

    COOL? You want cool?
     

    Attached Files:

  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Experienced DF'er's know that Nulls are much more sensitive (and discernible to the human ear) than peaks when taking Lines Of Bearing (LOB).

    Attenuators are helpful. Alternatively tuning 'Off Frequency" can be used to reduce the signal level.

    Antennas are antennas. There is no such thing as a "Super DF Antenna". shrug.

    Unless you are working major distances (miles) or with extremely weak beacons, I have generally found a two element yagi to be suitable.

    When working close (last 100 Feet, perhaps) in it can be handy to have a rubber duck to reduce sensitivity and avoid hangups on trees, shrubs, structures. A small rubber duck can be used with Body Blocking to obtain LOB's in-close (again, Nulling).
     
    K4YNZ and W6KCS like this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Using an FM receiver (typical HT) for bunny hunting is the pits. Once the signal level at the receiver input reaches about 1uV ( -107dBm), the limiters in an FM receiver IF remove any ability to detect small signal-strength changes beyond 1uV.

    I have a couple of receivers that were designed from the get-go to be used for radio-direction-finding and bunny-hunting. These receivers have an AM detector, and instead of FM IF limiters, they use AGC-controlled IF stages... This makes it possible for this type of receiver to provide useful 0 to 10dB S-meter readings for absolute signal levels ranging from 0.05uV to >1000000uV (yes, I mean 1V!).

    If you want to win Bunny Hunts, you need to get one of these:
    http://www.foxhunt.com.au/2m_sniffer/manual.htm
    Fantastic design...

    I have years of experience with tracking 121.5MHz Aircraft Emergency Locator Beacons. The most useful receiver for that application was also an AM design with AGC applied to every gain stage in the receiver chain (Rf amp, mixer, and all IF stages). Looked like this:
    upload_2021-3-26_12-20-6.png
     
    Last edited: Mar 26, 2021
    KA0HCP likes this.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

  7. KK4NSF

    KK4NSF Ham Member QRZ Page

    If you are interested in Fox Hunting, you need to take a trip down here to Mobile. Our local club has a growing group of fox hunters, and a number of excellent antennas / rigs.

    I'm not an expert by any stretch, but here is what I know about the subject based on my own RDF rig:
    1-I agree with the person above who mentioned that nulls are more pronounced than peaks. My own 2m yagi has a very sharp null directly behind it, but a relatively wide peak. Even so, no antenna will give you an exact line to the fox. It will give you thin pie shaped line that gets wider as it gets farther away.
    2- attenuators are important once you get within a few hundred yards of the fox. There are a number of designs on the Internet. I've had excellent results with a simple resistor / pot type, but others also work well.
    3- an S-meter is very important, as is the ability to easily turn down the antenna gain. Many newer radios do not have these features....or they are hard to adjust in the field, so I use an older Kenwood rig.
    4- I don't like tape-measure yagis. Their only advantage is in portability, and they have some disadvantages. I use a yagi that sort of folds up like an umbrella for transport.
    5- Fox hunting is a lot of fun.... but it can be harder than it looks. Some of the old gurus make it look easy, but when you first get into it, it can be frustrating.

    And that's all I know! Have fun.... and if you have the opportunity, come to one of our hunts. We're looking at having one in the next few weeks!
     
  8. WB8NQW

    WB8NQW Ham Member QRZ Page

    Check out the Fox Hunt Offset Attenuator by KC9ON. It will null out a 5 watt transmitter at less than 10 feet away.
     
  9. KO7PAS

    KO7PAS Ham Member QRZ Page

  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    One of the cardinal rules of antennas is that NULLS are always sharper than LOBES. The best DF antennas always use some sort of nulling function. Check out some of the old QST articles on 80 meter fox hunts. Nulling loops work just as well on 2 meters, which is why I never understood why everyone uses Yagis.
    And yes, I'm more than happy to model antennas....it's what I do. :)
     
    KK4NSF and KO7PAS like this.

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