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OCFMasters White Paper: Different Cores for End Fed Transformers

When feeding a half wavelength dipole from the end, we are presented with a very high impedance. It is necessary to transform several thousand ohms to match our 50-ohm transmitter. The design of that transformer is the most challenging aspect of the design. Other design challenges include counterpoise design and loading the dipole to align the different harmonic bands. Those challenges have straightforward solutions. We found that the optimum configuration was to use a 36:1 transformer and a counterpoise length of 5% of the wavelength of the lowest supported frequency.

The purpose of the transformer is to transform the high impedance at the end of the wire, 1800 ohms for the 36:1 transformer, to 50 ohms to feed our rigs. The challenge is to minimize losses and to maintain as close to a 1:1 SWR across the full width of the HF spectrum as possible. Minimizing losses is important for two reasons: First, power loss reduces the efficiency of the antenna. Second, and more importantly, lost power is dissipated in the core, causing it to heat up. When it reaches its Curie temperature, it stops working as a transformer and the SWR increases dramatically. It will work properly again after cooling off.

Our article in CQ, December 2021, discusses the process we went through finding the combination of popular cores that would meet these goals. However, the quest for the “perfect solution” doesn’t and must not stop. So, we kept looking for something better than the transformers described in our CQ article.

The most promising solution found thus far for End Feds with power levels up to 1 KW came from a suggestion from one of our readers, K9NUD, who mostly operates QRP! He suggested we try a large “bead” EMI reduction from Fair Rite, 2643251002, spec sheet below.

2643251002 Core

This core has a .66 inch inside diameter. That’s more than enough to accommodate RG-8 type coax and a few turns of #18 wire. Though it doesn’t give an AL, it does state that the impedance at 10MHz is 110 ohms giving an AL of 1,700. Measured at 100KHz as 3,000 seems about right. Either number will deliver adequate primary impedance with two turns. 

Next, we measured core loss by connecting two 36:1 transformers back-back with a VNA. The loss profile was nearly flat from 3.5MHz to 30MHZ and half of the loss in 240-43 cores that we rated at 500 Watts for two cores in our End Fed article. The led us to rate one core at 500 Watts at 50% duty cycle. We also ran tests into an 80M End Fed and found we could run a single core transformer at 250 watts Key Down for a minute. We also ran a 2-core version at 500 Watts Key Down for a minute leading to a 1KW rating at 50% duty cycle. What does that mean for real operation? The following are the ratings for the two core configurations.

 Mode Single Core Dual Core
Digital 150 Watts 300 Watts
CW 300 Watts 600 Watts
SSB 400 Watts 900 Watts

For comparison purposes we measured the electrical parameters of a stack of 240-43 cores, single 2543251002 and a stack of 2643251002 cores and measured the SWR curves of each into an 1800-ohm resistor. The values of the capacitor value on each of the primaries is also listed.

# Cores & Type Pri. Inductance  Sec. Inductance

Pri XL Ohms

Sec XL Ohms

1 240-43 8.58 uH 297 uH 180 6.35K
1-2643251002 12.85 uH 453 uH 283 9.96K
2-2643251002 26 uH 923 uH 572 20.5K


2-240-43 56pF 1-2643251002 56pF 2-2643251002 82pF

Currently a transformer with a stack of 2643251002 cores is in constant use at W1IS.

Photos of the single core transformer are below. The tape on the core is a temperature sensing tape we use to determine how hot the core got in operation.


This effort continues as we have more promising cores on order that we will test.

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