devicelab
Whacker Extraordinaire
I used Caddock MP930 resistors in series (2450 ohms) across the connectors of the Mr Baluns 49:1 EFHW balun. I ran a sweep on it at work:
Mr Baluns 49:1 EFHW Balun -- VNA Sweep
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I find different mfrs 49:1 or 64:1 transformers measure quite differently. You might scrounge up a MyAntennas version and I think you will be surprised how well it works.
devicelab
Whacker Extraordinaire
Yep I've been reading up on those but the MrBaluns 49:1 was 1/5th the cost the MYANT kit and 1/2 the cost of the BalunDesigns 49:1 -- although it technically supports up to 1KW output. I'll never go that high however.
You can make one with a single FT-240-43 or 61 mix ferrite toroid for about $10 plus wire and a disc cap around 125 to 150pf.
devicelab
Whacker Extraordinaire
prcguy, i purchased a 49:1 TennTennas transformer and saw something interesting. (I'll post its sweep later tonight.)
The primary connect point is super sensitive. If I simply place the plastic box on top of the VNA it goes nuts. If I put it anywhere near metal it goes nuts. If I touch the nut it goes nuts. I didn't remotely have this behavior with the MrBaluns model. To get the sweep I had to place it a good 2ft away from the VNA cart. Is this normal?
PS. The chart looks similar but this transformer favors the middle bands.
The primary connect point is super sensitive. If I simply place the plastic box on top of the VNA it goes nuts. If I put it anywhere near metal it goes nuts. If I touch the nut it goes nuts. I didn't remotely have this behavior with the MrBaluns model. To get the sweep I had to place it a good 2ft away from the VNA cart. Is this normal?
PS. The chart looks similar but this transformer favors the middle bands.
Hard to comment without seeing the setup. A 49:1 transformer connected to a VNA with coax and a resistor bridging its output to ground should be very stable. I've even measured two transformers back to back by for insertion loss connecting the outputs together with a wide ground strap between the ground side of the connectors. You might try a good 1:1 choke balun right at the transformer incase the coax is not decoupled very well from the transformer.
Ubbe
Member
When it's written 49:1 I assume the 49 side are the primary, or it would be written 1:49? But anyhow one side are high impedance and are then super sensitive to any nearby metal. That's why you have plastic or ceramic insulators on the end of antennas, that are high ohm, to isolate even to a tree branch.
You should see the same behavior from MrBalun but as the chart indicate high SWR it is probably of lower quality, attenuates more and depress the issue, or the length of the leads from the core of the 49 side are shorter to the point where you attach your load. The TennTenna balun are probably more effective but then also more sensitive to any mismatch.
/Ubbe
devicelab
Whacker Extraordinaire
Thanks Ubbe. Here's the chart for the TennTennas unit. I'll take an inside look later today. I'm curious to compare the designs between the two.
You might try different value load resistors like 2k, 2.5k, 3k and 3.2k as the response will be best when you find the best match for the transformer. That may settle down the top end a little and the cap across the feedpoint also targets the top end trying to cancel out reactance on the upper range of the transformer.
Thanks Ubbe. Here's the chart for the TennTennas unit. I'll take an inside look later today. I'm curious to compare the designs between the two.
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