You may find that when you change the existing balun for a new one, you will need to change the length of the radiating elements and feedline in order to achieve the same 'resonant' frequencies. Especially if you are substituting a Ruthoff 'Voltage' 'balun' for a Guanella 'Current' balun, or vice versa.
This is because the Ruthoff 'voltage' 'balun', which is commonly used for OCFD’s or 'Carolina Windom' has very little common mode current rejection. So the feeder becomes part of the antenna. In the case of the 'Carolina' Windom this is advertised as a virtue of the design. As radiation from the feeder is used to fill in some of the nulls in the polar diagram on the higher frequency bands. Providing a more Omni-directional pattern.
However when you use a Guanella 'Current' balun, with its improved common mode current rejection. The amount of feeder radiation may be reduced, and as a result the impedance measured at the base of the feeder section will also be modified. As the feeder plays less of a part in the overall radiating structure.
There are some things to watch out for here. The purpose of using a balun is to minimise current imbalance on the feeder, reduce common mode currents from flowing and hence minimise feedline radiation. The best way to do this is to use a balanced antenna to start with, then improve it still further by using a Guanella 'Current' balun.
When you use a Guanella 'Current' balun with an OCFD a lot of additional stress is put on the core material, because such a large amount of common mode current is already present. So if you intend to use the antenna with a tuner, or on bands where the resistive choking impedance of the balun is low. Make sure you choose a design with plenty of margin in terms of power handling.
This is why many commercial OCFD antennas and antenna tuners use a Ruthoff 'voltage' 'balun' as the lack of common mode current rejection (which is why I hesitate to call such devices a 'balun') means that the stresses the balun core are subjected to are much smaller. Making such devices almost indestructible. Especially if they are made using Iron powder core material. This offers very little resistive impedance and will recover if overheated, unlike ferrite core materials.
It is very difficult to design a Guanella 'Current' balun with a sufficiently high value of resistive common mode choking impedance. Greater than 1K Ohm (ideally it should be at least 5K Ohm) across the required frequency range of say 1.8 to 30MHz. If you are lucky you may achieve 5K Ohm across about 1/3 of the range and 1K Ohm over the remaining 2/3. Personally, I've only just about been able to achieve this in practice after a lot of difficulty. But I've never been happy with the performance of several OCFD's I've used them with. As I've not been able to achieve sufficiently good enough common mode current rejection to turn what is inherently an unbalanced antenna into a balanced one.
My existing antenna is a 90' doublet with 450 Ohm line feed, and a 1:1 Guanella 'Current' balun and auto-tuner at the base of the vertical feeder section. I can quickly add an additional 45' section to one side of the doublet, by means of a jumper cable. This converts it into a 135’ OCFD. This theoretically should have better performance on the low bands. However 99% of the time I leave it configured as a doublet. As the RX noise floor on the LF bands is much lower with the balanced antenna, and the very slight (about 1dB) increase in TX gain when configured as an OCFD is not really noticeable.
However you mileage may vary……………
Martin - G8JNJwww.g8jnj.webs.com