your reaction to this phase

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8GTR, Nov 30, 2019.

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  1. KC8GTR

    KC8GTR Ham Member QRZ Page

    Got a little bit of knowledge under my belt and have three working home brew yagis to show for it .
    Of all the reading and web searches , a lot of emphasis has been placed on efficiency , losses and gain in the VHF , UHF bands .
    There is plenty of great advice to make sure I'm running the most efficient station for every dollar / watt .
    But...no where have I seen a simple comprehensive article that mentions how much " reactance " is considered safe and / or negligible in regards to antenna loads ?
    Seems most impedance's will always have some reactive component but how much is too much ?
    Should I be concerned over a few ohms ( i.e. 2, 5, 7 ect... ohms ) ?
    There may be some reactants stemming from the use of UHF connectors or the coax itself , hard to tell .

    The 2 meter beam has a bandwidth of almost 800kc wide before the VSWR begins to slowly rise .
    By design and verified at final tuning at time of installation , I was able to achieve near perfect VSWR with reactances in the 1~5 ohm range when measured using the newest Comet CAA-100 meter .
    The reactance range mentioned is between a narrow VSWR measurement of 1.0 up to 1.2 . at 800 KC wide .

    Should I be concerned about a few reactive ohms ?
    I plan on purchasing a 1 kw linear amplifier for 2 meter soon and sure would like to see all 1 kilowatt go out the antenna .
     
    AK5B likes this.
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Your biggest loss on 2M will be with the coax you are using. A good SWR under 1.2 to 1 is very good, even a little higher than that would still be a very efficient antenna, from an SWR perspective. I assume the antenna and its matching network are rated for 1KW. How long is the feedline, and what are you using?
     
  3. AK5B

    AK5B Ham Member QRZ Page

    A few ohm's worth of reactance is usually about as good as you're going to achieve in the real world. Good for you for asking lots of intelligent questions and building your own Yagis just starting out, too.

    73,

    Jeff

    I will envy your kw on 2m as well! Long live VHF/UHF weak-signal work!
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    In a well designed rig that's stable into varying load impedance the main issue becomes power protection circuitry and foldback at high SWR. Sure, it's possible that a poorly designed rig or one with output circuitry problems could be unstable when operated into various reactive loads but that's not a common issue with commercially designed equipment when operated within their power foldback limits.

    Rig protection circuitry that manages transmit power foldback doesn't explicitly measure reactance nor really care about reactance by itself. Generally speaking these circuits sample reflected power with a signal fed back to the rig's drive stages to reduce overall output power as reflected power climbs. In essence they're measuring SWR via reflected signal not really measuring reactance by itself.

    So the key to delivering full power from the rig and not triggering power foldback circuitry is to manage overall SWR which of course is both the real and the reactive component of the impedance seen by the rig. A lot of rigs are designed to deliver full power up to a 2:1 SWR but targeting a system SWR that's a bit less as seen by the rig as in 1.8:1 or lower is good practice.

    As far as the transmitter is concerned any complex impedance that results in the same SWR will be treated the same from a power delivery standpoint. For instance any complex impedance that lies on the circle of constant 1.4:1 SWR will be the same as far as the rig is concerned. Here's what that circle of constant SWR looks like on a 50 ohm Smith chart with a few of the impedance combinations that can generate that SWR:

    upload_2019-11-30_9-30-47.png

    You could build a similar chart for the limiting SWR of your rig prior to power foldback (e.g. 2:1) and anything inside the circle would be an acceptable impedance as far as the rig is concerned. As shown above the highest acceptable reactance for a given SWR occurs where the real portion equals 50 ohms. So for the case of a 2:1 SWR limit the highest acceptable reactance would be approximately +/- 35 ohms but only when the real portion of the load impedance is very close to 50 ohms.

    So no, you won't see a lot of specs telling you how much reactance can be tolerated but you can find specs on limiting SWR for most rigs which of course takes reactance into account.
     
    KI4ZUQ likes this.
  5. KC8GTR

    KC8GTR Ham Member QRZ Page

    Hello ,

    I'm pretty sure the antenna can handle the 1 Kw or so .
    The simple gamma match uses teflon tape wrapped as bands around the matching rod for the co-concentric axial spacing AND for the high voltage insulating properties .
    Having an arc here is a really bad thing .

    I over engineer the whole antenna mainly to with stand tornado level winds .
    Tower is also doubled up to the 22 foot level than it's just the stock steel sections to the 38 foot top .
    Long story about the tower , I'll explain later , else where .
    Hopefully this will be the last time I'll ever have to fiddle with towers and antennas because of weather caused problems .

    The coax is all new , Davis RF flexible type from Texas Towers .
    Good folks at TT and still hanging in there after all these years .
    The Davis RF coax is tough stuff I tell you what !
    Good enough to compare with the LMR-400 flex without the price gouging .
    The coax run for the 2 meter beam is around 98~100 feet .
    So losses in the coax is well under 2db 's .
    Only thing I did not do and that is to coil up a few turns near the feed point to choke out any common mode currents running down the coax shield .
    So far have not noticed any serious detrimental effects of a skewed beam pattern .
    Finally down to the choice of connectors , I belong to the crowd whom can not fathom 1 kw being squeezed down to that itsy , bitsy pin of a " N " style connector .
    I used SO's and PL's through out all my antenna designs .
    The larger , robust pin of a PL-259 can accommodate the whole diameter of the inner conductor .
    With my years of experience soldering stuff , I can solder a PL-259 plug on in mere minutes .
    So it's a trade off... loss versus viability and convenience .
    My best guess with all known factors included , my station upon keying is about 95~98 efficient .

    Wish Kenwood would produce an upgrade to the TS-790a tri-bander radio .
    A good stand alone satellite rig upgrade would be very appreciated by me .
    High sensitivity / selectivity and state-of-the-art digital IF processing VHF / UHF / SUHF rig would peak interest with the microwave crowd and weak signals folks too .
    With built in rotator control [ signals ] perhaps ?
    Auto " hands free " frequency shifting for Doppler effect would be nice too .

    Not impressed with the TS-2000 radio though .
    Don't need or have the room for more HF band and VHF / UHF radios combo's .
     

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