Variable Capacitor for an End Fed Half Wave Transformer - Is it Worth the Effort?

Radio amateurs typically put in a 100 pF high voltage capacitor across the primary of an end fed half wave transformer.  I had the opportunity to test the range between 60 pF and 180 pF using a polyvaricon type of variable capacitor.

The results are as follows:

Fully CCW is 180 pF fully CW is 60 pF

On 40m the 2:1 SWR bandwidth is slightly improved from 370 kHz to 390 kHz

The SWR dip is also slightly better at 180 pF as compared to 60 pF which went from 1.45 to 1.38 

On 20m the 2:1 SWR bandwidth was improved from 615 kHz to 645 kHz

The SWR dip was also improved from 1.3:1 to 1.09:1

On 15m the 2:1 SWR bandwidth was improved more dramatically from 0 kHz  (the 15m band was above 2:1)  1155 kHz

The SWR dip was also much improved from >2:1 to 1.02:1

The 10m band was hovering just above 2:1 SWR

My standard QRP transformer on a recent build with the 100 pF capacitor measures as follows:

40m 580 kHz 2:1 BW with a dip at 1.8:1 7.00 MHz

20m 760 kHz 2:1 BW with a dip 1.03:1 14.14 MHz

15m 960 kHz 2:1 BW with a dip 1.2:1 21.4 MHz

10m 970 kHz 2:1 BW with a dip 1.5:1 28.8 MHz

Moreover, checking the SWR with a radio configured for field operation with the ATU bypassed with an 8 foot RG174 feedline.  The following are the results using the SWR bridge in my KX2

The 40m band is usable from 7 MHz to 7.25 Mhz with the lowest dip at the CW end at 1.5:1

The entire 20m band is < 2:1 with SWR being 1.5:1 at 14 MHz dropping to 1.2:1 at 14.35 MHz

The entire 15m band is < 2:1 with SWR being 1.7:1 at 21 MHz dropping to 1.0:1 at 21.45 MHz

The 10m band is usable between 28.68 MHz and 29.6 MHz with a dip at 1.7:1 at 28.8 MHz

So indeed the "goldilocks" compromise for the transformer is with a standard fixed high voltage (1 kV) 100 pF capacitor.  Fine tuning it with a variable capacitor is probably not worth the effort.  Furthermore, there is a penalty in transformer loss in the 10m band

MTR4Bv2 - How I pack it for for activations

The key is stowed underneath the black steel spring plate on the right (above).   The battery is on the left side. 

Below:  the phone is attached to the QuadLock while the GM0EUL key is fastened via its magnetic base.  The radio can be used without taking it out of the Pelican case.  The phone is running the OutdLog app for iOS for logging in contacts.

 

SOTA Presentation made to the Long Island CW Club

 

Thand to Bob Conder K4RLC for inviting me to present.   Dimitris Paliannis N1ED was the moderator and led the Q&A session.

Head to head tests - Less Efficient Core vs More Efficient Core - Using WSPR

The setup is as shown in the image below. The two antennas are arranged as inverted Vee in the same azimuthal orientation with the feedpoint on the south side denoted by the blue boxes. The image shows 10 foot topographic contours and also shows the global imagery to show both the presence of roads and vegetation. The two antennas are separated by about 75 feet of space. The NY4G coupler has the 2 turns primary and 14 turns secondary with the crossover and bifilar winding of the primary. The VY2AJ coupler has the 3 turns primary, 24 turns secondary with a tap instead of the bifilar winding and the turns are wound tightly together with no crossover. Both transmitters are identical 200 milliwatts WSPRlite programmed with their respective call signs. The only variables are the couplers and the relative position of the antennas with each other. They both see the same instantaneous propagation conditions. 

During the first day - NY4G was in the position of VY2AJ and vice versa.   During the second day, the positions were swapped to correspond to the image above.  The results from the first day below:

The propagation conditions varied throughout the day.   There are more watts coming from the more efficient transformer yet the less efficient transformer is keeping up with the more efficient one early in the day.  One attempt at an explanation is that there is a positional advantage from the position of NY4G on the first day - perhaps a 'yagi" effect for which the other antenna is acting like a parasitic element.  As the sun sets over Europe, with less propagation, the more efficient transformer can still hit the distant stations and the less efficient one, not as much.

During the second day, the positions were swapped to determine whether the "yagi" effect is for real.

The more efficient transformer (blue trace) hit more distant stations for most of the 6 hour period.  The advantage however is seen to be enhanced by the parasitic effect of the second antenna higher on the slope.  The only way to isolate this effect is to run identical transformers with the same setup.

With the same coupler, the lower position on the slop appears to have an 800 km advantage in reach over the one that is higher on the slope of the lot nas indicated by the DX10 Table for the entire 6 hour period.

NY4G is at this lower position. If one deducts the 800 km advantage from the previous two days results to sort of “equalize” matters from the “advantaged position” then

the average distance for Day 1 - reported by DX10 Table with adjustment
Coupler 1 VY2AJ (3 turn primary) - 5649 km Position 1
Coupler 2 NY4G (2 turn primary) -4751 km Position 2

the average distance for Day 2 - with adjustment
Coupler 1 VY2AJ (3 turn primary) - 10080 km Position 2
Coupler 2 NY4G (2 turn primary) -5509 km Position 1

the average distance for Day 3 - data reported by DXeplorer
Coupler 1 VY2AJ (3 turn primary) - 9047 km Position 1
Coupler 2 NY4G (3 turn primary) -9797 km Position 2

This suggests that the advantage of the more efficient coupler in terms of distance ratio is somewhere between 20% to 80% better reach.

Stay tuned for similar tests on the lower bands.

Owen Duffy Blog Post - Efficient End Fed Half Wave Transformer

https://owenduffy.net/blog/?p=12642 

I wanted to record the URL for this important blog article by Owen Duffy on autotransformer 

Efficiency Comparison between Transformer Cores with 14/2 and 24/3 winding ratios

 Measurements of the efficiency of the transformer cores have been made using the NanoVNA H4.

The results are documented here:

https://www.dropbox.com/sh/y4mb9xnwbn70hll/AACghPO0l2TF4UOuJAwyAGv6a?dl=0

The NY4GEFHW Store is live! ny4gefhw.com

 The URL is ny4gefhw.com

Please take a tour and browse around.

Trapped End Fed Half Wave - What is the Price of Convenience?

The thirty meter band is such a "money band" for SOTA activations that it simply can't be ignored.  Traditional end fed half waves either ignore it or put in a link to enable that band - e.g. the EndFedZ MountainTopper by LNR Precision (now Vibroplex). 

It is possible to configure an end fed wire with two traps, one for 20m and the other for 30m.  When finished, the inductive loading of the two traps shortens the antenna from 66 feet to 58 feet.   The bandwidth for 20m covers the entire band.  The band width for 40m is reduced but still perfectly acceptable within the CW portion of the band.  There are losses associated with this trap - the combined losses from both traps is somewhere between 0.3 and 0.7 dB.

I used it during my activation of Grassy Ridge Bald W4C/EM-001 as documented by this YouTube video and it worked fantastically well with my LNR MTR4Bv2 and GM0EUL paddle.

Here is an image of the end fed half wave antenna.  Only one trap is visible.

 

Is it worthwhile to use an RG8X Feedline for SOTA?

 I was just going through reflection loss calculations and comparing efficient and inefficient transformers.

On 60m, inefficient transformer are only 58% efficient.  Even with low SWR - say 1.5:1, the full entitlement available radiated power is only 2.78 watts.  

Compare that with an efficient transformer at 83% efficiency - radiating into an SWR of 6 but matched with an ATU, the radiated power is 3.77 watts.   Full entitlement with no coax losses is 4.15.  one would not want to go through this exercise with an RG174 feedline as the coax losses are worse - available output down to about 2.3 watts.  With very low SWR say 1.2, you are almost at full entitlement even with RG174.

So the lesson learned is - have a resonant antenna, however, in the case of mismatched antenna use the RG8X feedline instead of the RG174 and it always beats the inefficient transformer.

Having said all that, all this efficiency talk can be dominated by antenna configuration for an intended purpose.  Let's say the intended purpose is to work close in chasers on 60m.  An inverted V with an Apex at 20 feet has close to a 4 dBi  at a high take-off angle as opposed to the same end fed wire set up as a vertical (as impractical as that may seem).  The vertical will have a null at the high takeoff angle.  So an efficient transformer even with low SWR will not work any stations and that inefficient transformer will work those close in stations all day long.   There are a lot of factors that have to be optimized for effective antennas and as always - it always pays to begin with the end in mind and work backwards.  What is the purpose, configure the antenna for that purpose and optimal gain, and then worry about SWR second and last transformer efficiency.  It isn't easy.  As often, summit conditions dictate antenna configuration - presence or non-presence of supports and it becomes a "one size fits all" or "one antenna configuration fits all" proposition - the ubiquitous inverted V.

 The "SOTA Package" antenna includes 30m and 60m.   

Base antenna is a 40m half wave with a powerpole connector at the end for any extensions desired.

Options still include link for 17m, 30m, 60m extension, 80m extension 

Gen 3 of the NY4G End Fed Half Wave (Ariel's Aerials)

 

I have added both the 60m and 80m extensions to the system

The 60 meter extension is particularly suited to the narrow allocation on 60m between 5.3 MHz and 5.4 MHz.  The SWR is at or below 1.5:1 for the entire 60m allocation.  See the image below:

Weight of the entire antenna is 316g with the 60m extension - or only 53g heavier than the base antenna with 40-6m coverage.

Testing of the antenna using the Reverse Beacon Network on December 26, 2020 showed that the second harmonic of the 40m half wave has a better signal than the third harmonic of the 80 half wave as seen on the 20m RBN hits at 1527 UTC on 14.061 MHz (third harmonic) versus the ones at a later time on14.065 and 14.067 MHz (second harmonic).  RBN S/N ratio on both the 60m and 80m extensions were quite good and comparable to the 20m S/N ratio.  One can easily be heard on the 60m and 80m extensions.

The band width on 80m is quite narrow - with the 2:1 SWR band width being about 100 kHz wide and the extension must be carefully tuned for the rig to be employed.   If you are a recent buyer of the LNR MTR4Bv2 or the YouKits 7 Band 2020 version (https://www.youkits.com/products/youkits-hb-1b-mk5-2020-4-band-qrp-cw-transceiver-fully-assembled-and-tested), these extensions would be perfect.

Contrast the above image with the full size 135 foot EFHW Inverted L shown by the image below - shows that the shortening of the antenna has reduced the bandwidth by a factor of 3.  The KX3 being my shack radio has no problem tuning the antenna in the upper portion of 75m in the phone portion

Gen2 of the NY4G End Fed Half Wave

 

Generation 2 with the following improvements:

Winder is separable from the transformer so it can be replaced if damaged or if one that is impervious to weather is desired, a plastic one may be substituted

The end caps are now made of aluminum instead if walnut.   This gives more ruggedness to these parts.

Picture shown below is during testing and pruning for best balance across the bands.

Walnut winder and transformer box

88 feet of wire in the air for 60m operation.  The 60m extension is separable so that the 40-20-15-10-6 can also be deployed - resonant on these five bands

Walnut transformer box and white UHMW winder

Pruning and VSWR scans - the following are typical from the bottom end of each band

to the top portion of the band

The 60m extension connected

Antenna for SOTA, POTA, and DXpeditions

Initial Prototype:  The initial prototype had a turns ratio of 14:2 (14 secondary and 2 primary).  It used a single 100 pF capacitor across the center conductor to ground.  The tests of this initial prototype after pruning for a best compromise resulted in:

2:1 SWR Bandwidth on 40 meters is 6.730 -> 7.530 MHz with a dip to 1.3:1 SWR at 7.060 MHz

2:1 SWR Bandwidth on 20 meters is 13.910 -> 14.806 MHz with a dip to 1:1 SWR at 14.360 MHz

2:1 SWR Bandwidth on 15 meters is 21.010 -> 22.070 MHz with a dip to 1.6:1 SWR at 21.610 MHz

2:1 SWR Bandwidth on 10 meters is 29.1 -> 30.8 MHz with a dip to 1.8:1 SWR at 29.900 MHz

On 10m it is not very usable unless one has an ATU.

The second prototype (photo below) used two 220 pF capacitors in series which technically by the numbers should be 110 pF.  My measurements of the capacitance was closer to 130 - 140 pF.  This had a substantial effect on making 10 meters usable without a tuner.   The results are as follows:

2:1 SWR Bandwidth on 40 meters is 6.820 -> 7.510 MHz with a dip to 1.3:1 SWR at 7.150 MHz

2:1 SWR Bandwidth on 20 meters is 13.990 -> 14.840 MHz with a dip to 1:1 SWR at 14.430 MHz

2:1 SWR Bandwidth on 15 meters is 20.950 -> 22.270 MHz with a dip to 1.1:1 SWR at 21.650 MHz

2:1 SWR Bandwidth on 10 meters is 29.1 -> 30.8 MHz with a dip to 1.3:1 SWR at 29.900 MHz

Being a link dipole for 30m - here are the results for 30m:

2:1 SWR Bandwidth on 30 meters is 9.730 -> 10.880 MHz with a dip to 1.0:1 SWR at 10.280 MHz with a SWR between 1.3 and 1.2:1 from 10.100 to 10.150 MHz

A third prototype had the following results (identical in construction to the second one) with the following results.

2:1 SWR Bandwidth on 40 meters is 6.820 -> 7.500 MHz with a dip to 1.3:1 SWR at 7.150 MHz

2:1 SWR Bandwidth on 20 meters is 13.980 -> 14.850 MHz with a dip to 1:1 SWR at 14.440 MHz

2:1 SWR Bandwidth on 15 meters is 20.780 -> 22.280 MHz with a dip to 1.2:1 SWR at 21.600 MHz

2:1 SWR Bandwidth on 10 meters is 24.5 -> 29.9 MHz (which actually overlaps into 12 meters) with a dip to 1.1:1 SWR which is wide - 28.0 to 28.5 MHz so 10 meters is now perfectly usable without a tuner and so is 12m which is a very narrow band.

This is one heck of an antenna - requiring no tuner on 40m, 20m, 15m, 12m and 10m and power handling ability as follows SSB 60w, CW - 40w and Digital (key down) - 30w.

The final product:

 

 

All Time New One (ATNO) #315. South Orkney Islands

This DXpedition took place between late February and early March 2020 just before the pandemic hit with full force.  Because of the pandemic, the major DXpeditions have been cancelled.  The one going to Midway was one I was looking forward to but became victim to the pandemic.   The last major one I was able to work was VP8PJ and I was able to make contact on 30m CW.  Between the sunspot minimum and the effects of the pandemic - I will likely be stuck on #315 for a long while.

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