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I have a Hustler 5BTV antenna i resurrected after a previous monopole succumbed to birds.

It is giving me a world of trouble trying to get it tuned for resonance.

For those not familiar with the antenna its a 5-band trapped vertical.

It is ground-mounted (about 1 foot off the ground on a 1.5ft metal pole) with 8 radials below it (insulated electrical wire) running flat on the ground in each direction. 270-degrees is concrete underneath, and 90-degrees is dirt (garden bed).

Directly below the antenna on the pole is a home-made 1:1 choke (2 x FairRite FT240-31's w/11 turns of RG-400 around both toroids). An SO-239 on one side, and 2 bolts, one to centre element of the 5BTV via a 2" wire pigtail, and 1/2 foot wire pigtail to connect to radials below.

Transmission line is 32M of RG-213.

Below are the results from a NanoVNA (from nanovna-saver)

NanoVNA calibrated at load/feedpoint overlaid over NanoVNA calibrated at source/generator end (in blue) with 1:1 choke & 5BTV, RG-213:

enter image description here

And the same chart for the 1:1 choke with 2x100ohm wire-wound resistors in parallel replacing the antenna, VNA calibrated for load-end:

enter image description here

(grey bars are the ham bands from 160M-10M)

I can get an acceptable tune on 80M (narrow-band as it's a coil), and 40M is ok on the low-side, but most other bands aren't usable (too high an SWR, mostly over 4:1 up to > 10:1).

I'm wondering if the SWR curve in the bottom-most image (choke) is what should be expected for 2xFT240-31's? Because the SWR of the choke rises on higher bands, i'm wondering if that's why i'm having trouble getting a tune on the higher bands?

Would appreciate any thoughts/insights..

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    $\begingroup$ Wirewound resistors are not a good choice for this frequency range.Their inductance may start to become evident around 40m and will become steadily more significant as you increase frequency. Garden-variety metal-film and carbon-film are more appropriate. $\endgroup$ – Brian K1LI Oct 6 at 15:33
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Directly below the antenna on the pole is a home-made 1:1 choke (2 x FairRite FT240-31's w/11 turns of RG-400 around both toroids). An SO-239 on one side, and 2 bolts, one to centre element of the 5BTV via a 2" wire pigtail, and 1/2 foot wire pigtail to connect to radials below.

I would guess the pigtails and balun are large enough to be significant, and the source of your problem. You can see from your second VNA screenshot that the balun is introducing some significant impedance. If it weren't, the Smith chart should just be a dot in the center, as if you put a 50 ohm load right on the VNA.

I've had a 4BTV, and initially tried installing it in a similar manner with a similar balun I used to use for a dipole, and I had a hard time getting it to tune even though I had no issues with the antenna in a previous installation.

Ultimately, I ditched the balun and made a bracket to mount an N connector right on the base. DX Engineering sells a similar, fancier bracket, but I just made my own from a bit of the copper strap I had for grounding, bending it into a box and brazing the corners. The objective is to get the connector very close to the bolt which connects to the vertical element. Then the radials attach directly to the antenna base, to which the coax connector is also bolted.

The coax ran from the antenna connector into a weatherhead on some conduit, which went about 80 feet buried just a few inches underground to the shack. The balun proved unnecessary. The coax looks just like one more radial, so the more radials you have the less common current will be on the coax. And since it's buried, and long, I figured there wouldn't be any significant common mode current at the shack end, which I confirmed with measurement.

If your situation is different (especially if you can't bury the coax and it's not very long) and you do need a balun (we better call it a common-mode choke, because there's no "balanced" in this system) then I'd suggest making a choke with coax connectors on both ends, and adding it in the middle of your feedline at some point at least 1/4 wave away from the antenna. Trying to choke the common mode current at the feedpoint can be futile because the coax wants to be a radial. Move the choke away from the feedpoint and you just let part of the coax be a radial (free efficiency gain!) and you avoid any possibility of affecting the tuning.

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    $\begingroup$ Phil I'd expect a choke made of coax wound around the cores to be have no impact to the signal on the cable. Though it will break the braid connection to the antenna which will change the antenna a bit. I agree it might not be needed. $\endgroup$ – tomnexus Oct 6 at 22:25
  • $\begingroup$ @tomnexus I don't think it's the balun per se, but the pigtails. $\endgroup$ – Phil Frost - W8II Oct 7 at 4:02
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Ran a little experiment to try what Phil mentioned, i raised the radial ring from ground-level to just below the balun, and used a shorter (3" or so) pigtail to connect to the radials as a test.

Pigtail between the choke centre-pin and and centre screw on the 5btv is exactly the same pigtail as before (around 3" as well..)

NanoVNA calibrated at load/feedpoint overlaid over NanoVNA calibrated at source/generator end (in blue) with 1:1 choke & 5BTV, RG-213:

enter image description here

So, the moral of the story is to keep the centre and braid connections short by either using a so-239 and a coax pigtail, or if using wires, keep them as short as possible.

Studying DX Engineerings PDF on the BTV this isn't mentioned explicitly, but one can notice all the intra choke/radial-plate connections are short, wether it's a direct so-239 to the btv itself, a bulkhead so239 on the radial-plate with coax pigtail, or a short tinned copper-braid from DX Engineerings own choke.

The proof is in the pudding :) Thanks to all who provided advice, this is a test setup, now i'll have re-design the pigtails more permanently, soon.

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  • $\begingroup$ Obviously the antenna is not tuned (currently using DXEs "Ground mounted with radials" dimensions as a starting point). Once i make the radial connection more permanent, there should be better 'dips' to tune for resonance, as was evidenced in this experiment. $\endgroup$ – t252 Oct 10 at 3:14

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