Whip Me Longer - A Few Thoughts on KX2 Handheld Antennas

Executive Summary

You can use a relatively long 6 to 8 foot non-resonant handheld whip with the KX2 bracket and KX2 tuner.  It might outperform a 4 foot whip by 5 dB or so.

Experiments showed that:

1) The KX2 tuner will tune a 7 foot whip to an SWR near 1:1 on 10 through 20 meters.

2) Adding a small Buddipole coil as a series inductor (about 6.4 uH) to the base of the 7 foot whip, the whip can be tuned to 1.3:1 on 30 meters;

3) Adding a large Buddipole coil as a series inductor (about 18.2 uH) to the base of the same  whip, the whip can be tuned to 1.6:1 on 40 meters; and

4) 60 meters was a bridge too far with the large coil and a 7 foot whip; and

5) Substituting a 6 foot whip for the 7 foot whip gave similar but marginally poorer results; and

6) These results aren't limited to whips tuned by a KX2.  Resonant operation of antennas like an MFJ-1898 slider coil antenna, or a Buddistick, benefits similarly from being almost double the length of a 4 foot whip.

What are we trying to do here?

If you're familiar with the AX1, you know it's a popular 4 foot BNC-mountable antenna that uses a base-loading inductor in conjunction with the KX2 tuner to give handheld operation of the KX2.

If you have a more robust antenna mount, it may be possible to use a longer whip and the same concept to provide a more efficient whip antenna for the KX2.

Theoretically, remembering all models are wrong of course, a 7 foot whip should have a higher radiation resistance than a 4 foot whip, more efficient by the square of the whip lengths.

I'll spare you the math, but this could result in an improvement of just under 5 dB.

From the last post, you've seen a 3/8-24 antenna mount for the KX2, in that case used with a relatively heavy but slide tunable MFJ-1898 antenna.

I used that same bracket for these tests.  The picture below shows that bracket, and the antenna jumper, fitted with a 16.5 foot drag wire that was used for all the tests.

You may ask why 16.5 feet, I read 13 feet is a good number.  

That's probably true, but I use this 16.5 foot wire because if I suspend the far end, it's resonant on 20 meters, and it is marked so that it can be rolled up and used as a resonant counterpoise on the higher bands as well.  

Over the years I've found this seems to work better for me if I can suspend the far end of the radial, and when I can't, I just unwind it fully to use as a drag wire. Doesn't seem to work any worse than my 13 foot wire, so this is how I roll, so to speak.

For the first set of tests I used the 7 foot whip from the MFJ-1898 with the KX2 tuner to see what results I could get.  I'll admit, I was a bit surprised.

This is the MFJ whip from the 1898 mounted (without the slider coil) to the KX2 bracket.  The whip is 7 feet long, and weighs 3.5 ounces.

In a nutshell, the tuner tuned the 7 foot whip to near 1 to 1 SWR on all bands from 10 to 20 meters.

Performance on 30 was not good, so I added a small Buddipole coil having a measured inductance of 6.4 uH.  No effort was made to resonate the coil and whip.  This yielded good performance on 30 meters, SWR about 1.3:1.

Similarly, the small coil and whip performed poorly on 40 meters, but when the larger Buddipole coil, measuring about 18.2 uH was added at the base of the whip, the tuner managed to tune the system to about 1.6 to1, again a very happy result.  

It was interesting to note that the tuner would not tune the whip and 18 uH series inductor to 30 meters.  Jumpering the 18 uH coil to about 6 uH solved that problem, so you could get away with a single coil.

As before, no effort was made to resonate the whip and coil combination, the coil is just being used as a series inductor so that the combination falls within the range of the most excellent KX2 tuner.

The Table 1 below summarizes the results of the test.

TABLE 1 -  7 Foot Whip Experiment

Band (mtr)         7 ft MFJ        with small BP Coil        with large BP coil

    10                        1.1:1                    NA                                NA        

    12                        1.1:1                    NA                                NA

    15                        1.1:1                    NA                                NA

    17                        1:1                       NA                                NA

    20                        1.3:1                    NA                                NA

    30                        3.8:1                    1.3:1, 1.5:1                   NA

    40                        9:1, 10.4:1           1.3:1                            1.3:1 tapped like small coil

                                                                                                 9:1 untapped (all coil)

    60                        NA                        NA                              6.6:1, 10.4:1

And What If I Don't Have a Great Tuner - I own an FT-817 or an IC705

The results will hold with a resonant antenna as well as one tuned by the rig.  

I've used the MFJ-1898 slider coil whip (about 8 feet in length) and a Buddistick (about 7 feet in length, using a Featherweight whip and a small coil on 10-20, the larger on 30 and 40), and had similar results. 

Yeah, it's annoying to tune them a bit, but I always manage to get to a VSWR below 2:1.  You'll need a 3/8-24 antenna mount, which you can find elsewhere in this blog.

Conclusions

So what have we got?

On the plus side.....

If theory is right, we might have an easily implemented handheld antenna with about a 5 dB improvement over a shorter antenna like an AX1 or a MFJ single band whip.

And that's with all other things being equal.  On the bands where the inductors are used, the Buddipole coils are constructed from what is probably lower loss wire, on a low loss coil form, where the coil has a greater width to length ratio - something often cited in the literature as a more efficient coil configuration.  So there may be some additional improvement from the quality of the coil used.

And between bands where a series inductor is not used, QSY is virtually instantaneous.

On the minus side......

The antenna components are longer.  The whip is about 11 inches collapsed, where an AX1 whip is a few inches shorter when collapsed.  

And the Buddipole coils are bulky. So again, if pocket size is important, there's that.

And you need the 3/8-24 bracket.  I would not want to put a 7 foot whip on the KX2 chassis mount even if I could find one.

Oddly, the weights aren't much different until you throw in the big Buddipole coil on 40 meters.

The MFJ whip weighs in at 3.5 ounces, just 0.3 ounces more than the published AX1 weight.

The AX1E coil for 40 meters has an advertised weight of 2 ounces. The Buddipole small coil weighs in at 3 ounces, and the larger coil at a heftier 5 ounces, and it's pretty big.

Of course the weight does increase with the MFJ slider coil, but as they might say, if you can hold it, you can work 'em. :)

The Bottom Lines

1) As always, the internal tuner on the KX2 is amazing.

2) If you're willing to trade a bit of physical size increase, you may be gaining yourself (sorry) 5 dB ish or so.

You can decide if you want to make that trade.  

I did exercise the 7 foot whip in the CQWW SSB Contest this morning for a bit. 

I worked a TM and a CR6 quickly on 10 with no difficulty.  Like the MFJ-1898 with its coil, the 7-8 foot antenna does perform well for a handheld. 

I'm liking it.

And if you must choose a resonant antenna instead of a whip tuned by the radio, you can get similar improvement - doubling the length of the radiator should still quadruple the radiation resistance, which with the lossy ground type of situation you have with a handheld rig, should quadruple the efficiency = 6 dB.

As always, these are my results, not recommendations.  

Only you can keep yourself safe. 

Remember the cautions about using a handheld whip safely, avoiding others, pets, physical hazards and electrical lines, and follow RF safety guidelines.

72  Scott ka9p/zf2sc

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