Question 1

Will this work for a balanced diopole to and unbalanced coax line ?

Accepted Answer

Alfred E. Parker's article is a good one, but his answer is misleading.  In fact, the MFJ-915 CAN be used as a 1:1 current balun at the feed point of a resonant center-fed dipole, though it isn't mechanically optimal.  With an SO-239 adapter on the output, it will work for temporary antennas, and I've used it that way, but the MFJ-918, with the same internal design, is better suited mechanically for a permanent antenna installation.  Other companies make similar devices, including Balun Designs, whose prices are a little better than DX Engineering, while their quality is sometimes better than MFJ.
This is what is going on here (trust no one on baluns, including me; they are notoriously tricky to design or specify for a particular application):  A center-fed dipole, at resonance, will have a purely resistive impedance of 40 to 50 ohms, depending on details of construction and mounting height above ground.  Since this is close to the impedance your radio and coax want, you need no impedance transformation, merely a balancing of current at the feedpoint, which is exactly the same thing as blocking the common mode current.  A common mode RF choke IS a 1:1 current balun.  It can be placed at other points in the feed line as well, leaving the coax shield past that point as part of the antenna system, which is sometimes desirable as a counterpoise for odd, unbalanced antennas.  A dipole that is NOT center-fed or not at resonance will have different impedance, and may require an impedance transforming balun, like the common 4:1.  If you think you need one of those, to feed an off-center fed dipole, or a Windom, or a ladder line, check out Balun Designs website.  They explain which 4:1 design you want for each of those applications.
A note about the impedance of a center-fed dipole: You may find that the SWR is minimum slightly off resonance, this is because the resonant impedance is not 50 ohms for your particular antenna in its particular environment.  Off-resonance, the resistive component of impedance can rise to nearer 50 ohms, and offset the increasing reactive component, to give a minimum SWR.  It doesn't really matter much, but minimum SWR is often NOT at resonance, where the impedance is purely resistive.  I know, this can drive a person nuts, I keep re-reading this stuff every time I work with a new antenna or modify an old one!
Be cautious of power rating.  Under some circumstances, depending on antenna design and feedline length, a balun will end up dissipating a fair amount of power.  Make sure you have the correct balun type for the job, and that it is rated for enough continuous power input not to burn up!

Question 2

Is this a low pass filter?

Accepted Answer

No. It consists of ferrite rings surrounding 50 Ohm coax. It serves to add inductance reactance (increase impedance) to both the shield and center conductor between the antenna and the transmitter. The common use is to remove RF from the case or ground of the transmitter that causes RF burns while transmitting. it does not have any impact on the SWR or attenuation of signals. Often the transmission line is exposed to the strong fields from the radiator elements of the antenna and the shield of the coax tends to present RF on the shield. My antenna is very close to the rig and after installing the isolator, all burned lips have healed. if i can be any further help, you can call me 469-231-2784.Jerry WestW5YXR

Question 3

Why are these so much more expensive on Amazon than they are everywhere else?

Accepted Answer

Vendors may have different pricing, as they may sell more or less of any device, but that is up to the vendors. Not sure if you can negotiate pricing.

Question 4

When you hook up the mfj swr analyzer to the antenna do you keep the if Isolator in line also while your reading the swr readings?

Accepted Answer

Yes, it becomes part of the antenna system.

Manufacturer	MFJ
Brand	MFJ
Item Weight	10.5 ounces
Package Dimensions	7.68 x 7.64 x 2.24 inches
Item model number	MFJ-915
Is Discontinued By Manufacturer	No
Manufacturer Part Number	MFJ-915

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MFJEnterprises Original MFJ-915 RF Isolator 1.8-30 MHz, 1500W PEP, SO-239

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