Dave Richards AA7EE

March 9, 2011

“Any Old Piece Of Wire Will Do” or “Why It’s Good To Forget What You’ve Learned”

Filed under: Amateur Radio,Ham Radio,QRP — AA7EE @ 8:23 pm
Tags: , ,

I miss making blog posts about building kits.  I think the last major one I did was about building the Tut80, and before that, the Norcal 2N2/40.  It’s been a while since those posts, so I’m looking forward to posting about the building and operating of Etherkit’s new CC-40 QRP CW transceiver.  Many of the blogs I follow are updated on a much more regular basis than mine, and I enjoy reading them, so although I don’t think I’ll be able to offer the content that many of my favorite blogs do, I can at least pass along my rambling ham radio thoughts while I wait for the beta kit of the Etherkit CC-40 to arrive at the AA7EE QTH.

The current antenna here is an inverted vee cut for 40M. The center is supported by a 32 foot fiberglass windsock mast that I bought from The Mast Company.  It’s mounted on the balcony of my house, which is about 15 feet above ground, putting the center of the dipole at about 47 feet.  75 feet of  JSC Mini RG-8/U type 3060 connects the dipole to my Z11 tuner, which is sitting right next to the FT-817. It’s cut for 40, so works great there. It also works on 15 – not sure how well as I haven’t had many QSO’s on 15 yet.

So far so good – a 40M dipole is expected to work on both 40 and 15, so no surprises there. Lately though, with the sunspots becoming postively more sociable, I’ve been wanting to be QRV on other HF bands too. At some point soon I will most likely try feeding a dipole with balanced line for all HF band coverage but right now I don’t want to mess with the coax-fed 40M dipole as it’s working great on 40, and I want it to remain that way for giving the CC-40 it’s on-air testing.

So what to do? I know that I can use the Z11 tuner (which is located next to the rig) to present an acceptable impedance to the rig so that it will transmit on any HF band, but I also know that on bands other than 40 and 15, the mismatch at the center of the dipole will be great, incurring losses in the coax. In other words, only a bozo who doesn’t know his antenna theory would try to load up a coax-fed 40M dipole on bands other than 40 and 15 with an antenna tuner located at the rig.

So I tried it.

And it gets out.

Amongst the QSO’s I’ve had with this “unwise and inadvisable” antenna (my quote marks) are FG5FR on 30M and LU4FLJ on 12M.  On 30M this morning, I QSO’ed with AB7KT who was running the same power as me – 5 watts (not sure what his antenna was). He gave me a 579 and I gave him a 559.  Also on 30M, WB7NZI was running 83W (odd figure) to a dipole.  He gave me a 559 and I gave him a 589. KD0V was running 100W to a 3 ele beam at 50 feet.  He gave me a 579 and I gave him a 589.  Remember that there is 5W coming out of my transmitter (not sure what the erp is).  There are many other examples, but I have had successful QSO’s on 40, 30, 20, 15 and 12  – all on an antenna system that my understanding of antennas dictates should only work well on 40 and 15.

Does anyone reading this know how I could model or otherwise calculate the theoretical losses for such an antenna on bands other than 40 and 15? I’d love to know what the theoretical losses are on this system on bands other than the one for which it was designed, because going on just signal reports alone, the losses don’t seem to be that great. Although I haven’t had QSO’s on 80 or 10 with it, I have seen my spots on the Reverse Beacon Network, and the SNR figures are quite encouraging.

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6 Comments »

  1. You didn’t say anything about a balun at the feedpoint of the antenna. If there is none, RF is getting on the outside of the coax at that point and your antenna is more than just an inverted V. Depending upon the length of the feedline, it could radiate quite well. I would predict that if there is no balun up there, and you install one, the RF on the outside of the coax would be reduced, and so would your number of QSOs on bands other than 40 and 15 meters. If you don’t install a balun, changing the length and/or position of the feedline might make a big difference on other bands, for good or ill.

    David, ex-W8EZE

    Comment by David Ryeburn — March 9, 2011 @ 11:12 pm | Reply

  2. Dave,

    Here’s a pretty good calculator to measure your losses if you know the SWR for the band of interest: http://www.arrg.us/pages/Loss-Calc.htm

    It has a second part that will tell you your ERP.

    From your descriptions of the QSOs on the “off bands” it seems to me that the other op’s power and antennas are doing the heavy lifting. Should be a snap to use David’s advice to find out if the coax is adding to your signal or not.

    See you on the Cloud Warmer

    73!

    Comment by Casey TI2/NA7U — March 10, 2011 @ 12:22 am | Reply

  3. There is no balun at the feedpoint David, so I imagine that the feedline has indeed become part of the antenna. If I were running more power, this would probably be a recipe for stray RF in the shack, TVI etc but as my station is QRP, I don’t have those problems.

    So when I’m in QSO with a station and we’re not on 40, what do I call the antenna instead of calling it “A coax-fed 40M inverted vee”. Would “random longwire be somewhat descriptive?

    Comment by AA7EE — March 10, 2011 @ 12:24 am | Reply

  4. The demo/free version of eznec will model a simple doublet.

    Last year for RSGB IOTA we used a pair of doublets slightly shorter than a 40m dipole. The legs were 29 feet, total of about 59 feet. That works well on 40/15 and 10m. (Didn’t model for 17 or 12).

    On 40m its still basically a dipole. On 15m, its closer to being an EDZ. No idea what it works out to on 10m, but the impedance is relatively easy to match.

    Was it radiating from the feedline? Maybe. The 1100+ QSO’s were logged anyway.

    73 es have fun de w4kaz

    Comment by w4kaz — March 10, 2011 @ 12:47 pm | Reply

  5. Antennas are funny beasts alright. Fun to tinker with though, because of it. I never did put up an antenna on 160 for this season so I force-fed my 80m dipole on 160, often with good DX results. The sun’s recent outbreak of acne is certainly helpful in your case…the “world on a wet noodle”, etc…

    Comment by John AE5X — March 10, 2011 @ 3:44 pm | Reply

  6. Hi Dave. I whipped up a little model using cocoaNEC on my Mac. I used three wires over average ground (0,002 S/m, e=10). One was from (X=-22, Z=24) to (-0.1, 47) feet; one from (22, 24) to (0.1, 47) and a feed segment from (-0.1,47) to (0.1,47). Values for Y were all 0. The input Z at 7.05 MHz was 84.4 + j3.4 (SWR=1.69 with respect to 50 ohms); at 10.15 MHz was 275 – j938 (SWR=69.7); and at 21.05 MHz was 103.2 – j44 (SWR = 2.53).

    Using the coax calculator at http://www.arrg.us/pages/Loss-Calc.htm with 75 feet of Beldon 8237, at 7.05 MHz the total loss is 0.39 dB; at 10.15 MHz is 6.42 dB; at 21.05 is 0.85 dB. So on 30 m your five watts is down to 1.14 W at the antenna. This assumes your Z11 is ideal and there is no current on the outside of the coax, both of which are definitely not true. The former will only add more loss, but the latter might not hurt so much since it probably still radiates at least somewhat.

    For the sake of discussion, I tried 450 ohm window line instead of the RG8. Now at 7.05 MHz the SWR at the antenna end is 5.43; at 10.15 SWR = 9.35; and at 21.05 SWR = 4.4. But the line losses are now 0.16 dB, 0.30 and 0.22 dB; a lot lower all around. Since the losses are so low the Z11 will see about these same SWR values at the input end of the line. Even so, I very much doubt the Z11 loss will be greater than the improvement from the window line.

    Comment by Elwood Downey, WB0OEW — March 10, 2011 @ 7:12 pm | Reply


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