HF chokes (W2DU baluns)
It is possible to build very effective HF chokes using the Ferroxcube toroids detailed below. An impedance of 500 ohms corresponds here to an absorbtion of 20db (relative to 50 Ohm). In addition these ferrites are very well suited well for building so-called 1:1 choke baluns according to the W2DU principle (see ARRL manual).
The W2DU balun can tolerate high power, and avoids the phase shifts that can occur with conventional baluns. The cores fit very closely around the coax, leaving only a minimal space. Only the cores used with Aircell have a little more play. The cores can best be attached to the coax using the shrink tubing I can supply. You can find a description of some cores and their specifications on my "Toroid cores" page.
You will see from the graphs that the impedance starts to drop off at around 14MHz. A fellow ham was so kind as to measure a choke with -3S4 cores on a professional Rhode & Schwarz test bench, which allowed for compensation for its characteristics. This demonstrated that the absorbtion between 3 and 30MHz is fairly constant. I would be pleased to quote you for manufacture of HF chokes and W2DU baluns to your specifications. (for example, 20 CST 9.5/5.1/15-3S4 cores on 45cm Teflon coax withit two PL259 plugs and a PL258 coupler pre-assembled and weatherproofed in shrink tubing for only 25 euro.
HF chokes using 5mm coax
Using CST 9.5/5.1/15-3S4 (15mm high) Ferroxcube cores allows an efficient and inexpensive choke to be built. The graph shows the impedance range of 20 cores on about 30cm of RG58 between 1.8 und 30MHz, measured with an AEA CIA-HF. More information can be found on my "Reducing interference" page.
The CST 9.5/5.1/15- 3S4 cores cost 0.45 euro each with a minimum order of 10 cores, 0.40 euro each from 50 cores up, 0.35 euro each from 250 cores up
Tip: HF chokes made on 5mm coax can be built for a lower price, and with far less bulk, than on 10.3mm coax. When made from short pieces of Teflon coax the extra losses incurred are negligible.
Accessories:
0.5m Teflon coax cabel, temperature resitant, 4.95mm diameter, Price 2.50 euro
0.5m shrink tubing, temperature resitant to 135c, flame resistant, Polyolefin (black) 1:2 shrinkage rate (9.5mm reduces to 4.8mm), Shrink temperature 120c, specially for Teflon coax, Price: 1.50 euro
(Teflon coax and temperature resistant shrink tubing are recommended for use with power amplifiers)
0.5m Ployolefin shrink tubing 1:2 shrinkage rate (9.5mm reduces to 4.8mm), quick-shrinking (from 80c), black, thin walled, NOT flame resistant, for use with RG58 or similar, Price: 1.00 euro
PL259 plug for RG58 or similar: 1.30 euro
PL258 (PL double female adapter) 1.60 euro
HF chokes using 7mm coax cable
(e.g. AIRCELL 7)
CST 16/7.9/14- 4S2 (14.3mm high) are used to make HF chokes on 7mm diameter coax, such as Aircell 7. The 4S2- mix has a lower permeability than 3S4, which results in slightly poorer results at th elower frequency range. Once again, I recommend the use of 20 cores.
The CST 16/7.9/14-4S2 cost 0.85 euro each with a minimum order of 10 cores: for more than 50 cores the price is 0.80 euro each.
Accessories:
0.5m shrink tubing, 1:3 shrinkage rate (18mm reduces to 6mm), thin walled, flame resistant. Price: 3.00 euro
CST 19/11/12- 3S4 (12mm high) are used to make HF chokes on 10.3mm coax cable. The graph shows the impedance (grey) using 25 cores on a length of RG 213.
The CST 19/11/12-3S4 cost 0.80 euro each with a minimum order of 10 cores: for more than 50 cores the price is 0.80 euro each; for more than 100 cores the price is 0.70 euro each
Accessories:
0.5m Polyolefin shrink tubing Polyolefin 1:2 shrinkage rate (19mm reduces to 9.5mm), quick-shrinking (from 80c), black, thin walled, NOT flame resistant, Price: 2.00 euro
PL259 plug for RG213 or similar: 1.20 euro
PL258 (PL double female adapter) 1.40 Euro
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