# Bandhopper 40m tuning

My 20/30/40 bandhopper appears to be tuned a little low.

This graph pretty much mirrors what the SWR reading on my FT891 is.

Any suggestions as to a starting point as to how much I should shorten each 40m link to get the center more around 7.150?

Thanks.

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If you want to lower the frequency length should be added. I would suggest starting with 4 inches and trim down as required, maybe a half inch at a time.

Source: 234/F = 1/4 wave
The length calculation for 7.15 is about 3 inches longer than 7.2. Therefore you need to add the difference, plus a little extra for trimming.

Good Luck - I have been there before and once cut what I should have added.

73,
Howard

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And donâ€™t forget raising or lowering the center mast thereby altering the angle of the inverted v can have an effect to a small degree on where the dip is.

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Thanks. Iâ€™m looking to raise the lowest SWR from 7.000 to around 7.150. I can use this as a starting point.

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Yep, Iâ€™m using a 6m mast when I use the antenna so the center is pretty much a fixed height.

Hi Richard,
The resonant frequency is also dependant upon the ground below the antenna. If you are putting this antenna and 6m mast up above rocks it will have a different resonant frequency to if you are putting it up where the ground is moist soil for example.

What you donâ€™t state is what are the 1.5:1 or even 2:1 SWR points on the band - if the SWR is less than 2:1 across the whole band, Iâ€™d leave it as is. Even better if itâ€™s 1.5:1 across the whole band. The amount of power you lose by the SWR being 1.5:1 as opposed to 1:1 is negligible and as you cannot control what the ground will be when operating portable a change in length that you make for one summit wont be right on a different summit.

73 Ed.

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As a guide: The length of a 40 m dipole changes by approx. 29 cm per 100 kHz. So to increase the resonance frequency by 150 kHz your antenna should probably be shortened by approx. 20 cm on both sides (advantageously, step by step, hi).

Edit
In order not to lose the possibility of later using the antenna with a longer mast, one could simply bend the excess length back (instead of cutting it off) and hold it tightly together (e.g. with insulating tape or shrink tubing).

Edit
Some of the comments above are obviously based on a misinterpretation of the screenshot: The resonance frequency is marked with cursor 1 (7.000 MHz) and not to be confused with the set center frequency of 7.200 MHz).

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You could make the 20m links longer. Use a length with connectors so that you can remove it if necessary.

?? That would be a very silly thing to do and would not achieve the result that he is looking for. I can only assume that you have misunderstood the OP.

Due to the larger relative apex height at 20 m (also at 30 m), the frequency offset on these bands with a (short) 6 m mast will be lower than on 40 m, so that no length adjustment should be necessary.

Am I being silly? He needs to lower the resonant frequency, involving lengthening the antenna by a few inches. Making the links longer would do this. The small catenaries that would result are tiny in proportion to the resonant length and should have no discernable effect on the radiation performance of the antenna. Making the lengthening pieces removable will enable him to remove them if ground conditions or the height of the antenna lower the resonant frequency. What have I missed?

Read my first posting Re: misinterpretation of the screenshot.

This. This a million times.

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Which requires the antenna to be shorter, not longer

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The display from that nano-VNA (or whatever) is confusing! Took me a while to realise the marker is at 7.0 not 7.2MHz As I read it the minimum SWR is at 7.0ish.

The last section of each side of the antenna is the section to be shortened, if you need to do it. The SWR < 2.0 is a good guide.

My advice is donâ€™t cut anything. Fold the end back, changing the point Where the wire goes through the end insulator. If you need to reduce the length by 18â€ť or 45 cm overall, thatâ€™s a reduction of half of that on each end of the antenna. Just fold the wire at that distance back from the end, making the fold the new â€śantenna endâ€ť and twist the excess length around the last 9â€ť of the wire, ie. going back towards the centre of the antenna.

I donâ€™t recall what the end insulators are on those antennas. I use bits of chopping board on some or just a loop with hoochie cord as the end guy. If you fold it back so that itâ€™s too short, itâ€™s trivial to lengthen the antenna then, just untwist, move the fold point and reattach to the end insulator.

Andrew VK1DA/VK2UH

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Yep, it took me a while to get used to it.

So what is the SWR at 7.15?