Theory
Windings
When you see a notation like 2:14 and 3:21 that means how many turns on the primary and how many turns on the secondary part of the transformer. 2:14 means 2 turns on primary, 14 turns on secondary. Likewise, 3:21 means 3 turns on primary, 21 turns on secondary.
So, where does the 49:1 come from? That is the transformation ratio. To arrive at it we consider the ratio of turns on the secondary to the primary. With a 2:14^{1} this becomes 14 to 2 or 7 to 1. Take the 7 and square it to arrive at 49:1. We’re transforming a roughly 2450 ohm feedpoint down to 50 ohms.
Other common ratios that are used with EFHW antennas are 36:1, 64:1, and 81:1. Respectively ratios of 6:1, 8:1, and 9:1. To make a 36:1 transformer you could have a 3 turn primary, and an 18 turn secondary.
Note: In these pictures, they are all 49:1 transformers.
Original 4S QRP Experimenter PCB 2:14
Original 4S QRP Experimenter PCB 3:21
2024 EFHW board with Double Stack FT8243 toroids, 3:21
2024 EFHW board with Single Stack FT8243 toroids, 3:21
Transformer Theory
The 100pF capacitor is often added to the primary side of the transformer in an End Fed Half Wave (EFHW) antenna for a couple of reasons:

Improves Higher Frequency UNUN Performance: The capacitor can help improve the performance of the Unbalanced to Unbalanced (UNUN) transformer at higher frequencies^{2},^{3}.

Compensates for UNUN Primary Leakage: The capacitor can help compensate for any leakage in the primary of the UNUN^{2}.

Flattens SWR at Higher Frequencies: The addition of the capacitor can help flatten the Standing Wave Ratio (SWR) at higher frequencies^{2}. This is particularly noticeable for frequencies above 20 meters^{2}.
It’s important to note that the specific impact of the capacitor can depend on the bands you are interested in. For instance, if you’re building an antenna for the 40m band, you might not need the capacitor. But if you’re also using the 20m band, the capacitor can be quite beneficial^{4}.
Please note that these are general observations and the actual impact can vary based on specific antenna designs and operating conditions. It’s always a good idea to experiment and see what works best for your specific setup^{2},^{4}.
Does the antenna system need a counterpoise? There is always one present. With end fed antennas you also need to watch out for common mode current (CMC) on the feedline.^{5}
Primary Source: Conversation with Bing, 3/20/2024
Antenna Length
An end fed half wave antenna is a halfwave length long and is fed from the end as the name implies. So, determine your wire length by first considering the lowest band you want to work on. For example, 40m, or around 7.030Mhz. Then use the standard dipole formula of 468 / frequency = feet. So, for our example, 468 / 7.030 = ~66.57ft or ~66 feet and 7 inches. As the adage goes it is easier to trim wire than add it. Therefore, I would start with ~70ft of wire and adjust back from there. A simple thing to try is to fold the wire back on itself at the end.
The normal dipole like radiation pattern for the EFHW is when it is being used on it’s primary band. You will have more lobes on the usable or harmonic bands. With more lobes the further you get from the primary band.
You will most likely need to use some small amount of tuning when using the antenna portable or on nonprimary bands.
Primary Band  Usable bands  Initial length 

160m  80m, 40m, 20m, 15m, 10m  265' 
80m  40m, 20m, 15m, 10m  135' 
40m  20m, 15m, 10m  70' 
20m  10m  35' 

https://noji.com/hamradio/pdfppt/noji/NojiArticle8010EFHW.pdf “8010 endfed halfwave antenna with 49:1 unun  Noji.” ↩︎

https://www.ai6xg.com/post/efhwxfrmrcapacitor “End Fed Half Wave Antennas: Is a Primary Capacitor Really Needed?” ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

https://www.ai6xg.com/post/endfedhalfwaveantennasmoreabouttheprimarycapacitor “End Fed Half Wave Antennas: More About the Primary Capacitor.” ↩︎

https://dxexplorer.com/491impedancetransformer/ “49:1 Impedance Transformer for EFHW Antenna  DX EXPLORER.” ↩︎ ↩︎

https://www.radiostuff.com/post/doineedacounterpoiseforyourefhw “Do I need a counterpoise for my EFHW?  M0VUE” ↩︎