Low Pass Filters

[W6HSA]

I noticed something interesting when checking two LPFs, a Bencher YA-1 and an ICE 421. I connected them both directly to the load side of my LP-100a into a dummy load and noted the ohmic, reactance and SWR readings. I tweaked them both to the same reading.

They both measured identically the same: 50 ohms, .04 reactance and 1.03 SWR into the load at the same frequency with the same signal source. When put in production; i.e., the LPF behind the LP-100a, they both registered the same constant carrier wattage, but the YA-1 passed more PEP with the same signal source. The PEP difference averaged a bit over 25% through the YA-1 over the ICE. Perhaps negligible on the receiving end but I found it interesting nonetheless. It appeared the average modulated power was pretty close which may be a more meaningful comparison.

My assumption is that the bigger coils in the ICE absorb more of the peak power than the YA-1. The power handling of the ICE is much greater and if this assumption is true, would work better with a much larger output.

Not sure if my assumption is correct so I thought I'd ask.

 

[Moleculo]

Coil design does make a difference in efficiency. I suspect that is what you're experiencing here, although I'm no expert on such things.

 

[W6HSA]

Thanks for that.

The ICE is a 9th degree filter. There is no description for the YA-1. The difference is in the coil layout. The ICE has two additional smaller coils along with the three main coils, which are common to both, and they are much heftier. It is very robust inside. The YA-1 is rated at 1.5KW constant and the ICE is rated at three times that much. The YA-1 is rated at 5KW PEP and the ICE is rated at 8kw PEP.

It think it is reasonable to conclude that power is being absorbed - actually it is the only thing I can conclude given both filters measure identically the same - by the larger coils. Again, I suspect at higher power, it would be more efficient.

Perhaps someone with more insight can confirm. I just thought it interesting.

 

[W6HSA]

Further for those interested:

Had a colleague whose SWR light on his base radio came on whenever he turned on his amp. We checked the SWR directly connected to the radio and found it flat with the amp off and 2.0 with the amp on. Ordered a 100 watt LPF, tuned it to precisely 50 ohms/.05 reactance/1.0 SWR into a dummy load (the readings of the dummy load) and the SWR light stopped. Re-checked the SWR with a meter, again directly connected to the radio before the filter, and it was flat in both instances. Now, all the power generated by the radio heads to the amp. In fact, the overall output of the amp is greater and the cooling fins on the radio were, well - cooler. The filter measured 40/13/1.9 before tuning!

Word got out and I've been ordering these filters from the vendor in Ohio and tuning them up for the group. Needless to say, he's happy. They are $14.85 shipped - can't beat them for what they do - and in every instance, base or mobile, they have flattened out the SWR between radio and amp and increased output. These amps do no have input tuning. The additional benefit is choking off the signal above 30Mhz so the amp doesn't have to work to produce them. I've even suggested they consider them after their amps as well. Heck, I even suggested ferrites. Dr. Selvig's writing convinced me of their efficacy, and I use W2DUs baluns, but that is another subject, I suppose.

Cleaning up the airwaves one at a time! :laugh:

Ahem, now then, back to the matter at hand. I took the big ICE and put it between his radio and amp and the PEP was less with it than with the 100 watt LPF. Same for the YA-1. This seems to confirm the larger coil theory. It appears sizing them for the application is important. I have a colleague who has a meter that will resolve sufficiently enough to measure the difference in the resistance of them all. I'll post my findings.

 

[Klondike Mike]

I can confirm similar findings with my LP100a. I have tested a few filters and have found that some cut power by as much as 40%.

 

[TonyV225]

I also have seen a difference in power with different lowpass filters and that is why its so important to have the filter inline when tuning an antenna sytem up because people would tune up and later add or change a filter not realizing they created a missmatch and threw a wrench in on their tuned system.

 

[W6HSA]

Thanks, guys, I hope others benefit from this.

To add a bit more to the pot, I have found that using a balun between radio and amp as well as just after the meter, blocks RF on the shield so the radio is happy and the meter reads correctly. I got this from Dr. Selvig's writing. It is most noticeable with RG8x but surprisingly with LMR400/9913. It seems there is always a bit of RF sneaking out, slipping through the braid since it is not 100% efficient, and playing subtle havoc on things. No neighbor complaints and a nice quiet receive. (I also have them at the feedpoints of my antennas). Extra expense admittedly, but a very efficient transfer of power and good reactance readings. To me it has proven to be good station design.

The LP-100a does not lie. Buying it was a real game changer.