I don't know/remember which FF4 version you built, whether 40-in or 48-in High Reflector. Either model indicates that it provides a very healthy 8+ dBi across Hi-VHF Band [rising to as much as 10+ dBi on Ch7 in the Taller Model]....but with miserable SWR:
First of all, I would recommend that you TRY adding various length Coax Extensions....the intention is to MOVE the SWR NULL Pattern along the Coax so that it does NOT occur on the END of the Coax at the desired Frequencies (NULLS occur every Half-Wavelength, which is 33.4-in on Ch7 and 28.5-in on Ch12). So adding a "standard" length coax of say 12-in and/or 18-in should CHANGE the NULL Pattern.
Another "simple" mod that you could try to see if 8 dBi Gain is "enough" would be to move the Reflector further away from the Bowties....such as 6, 10 and perhaps as much as 16-in away to Optimize for Hi-VHF performance at expense of UHF.
Instead of a Hi-VHF Band Yagi, you might want to consider one of the alternative "Hourglass-Loops" analyzed here, where you can choose how many Reflector Rods you want to build. Since Wire Size is NOT important, you COULD use AWG12 or larger....incl. the Metal Shield Wires in RG-58 or RG-59 Coax...with NO NEED to remove the Insulation:
If you find that you "need" even more Hi-VHF Gain, the fol. Horizontal Stack of UHF Hourglass-Loops (3x Bigger for Hi-VHF) illustrates what can be done that MAY provide 10+ db of Space Diversity Combining Gain [depending on severity of Multipath Fading] in addition to nominal 3 dB Summing Gain:
Obviously the K6STI 5-El Yagi will have low SWR in Hi-VHF Band....but it only provides Gain = 7.3 dBi on Ch7 and roughly same Gain as FF4 on Ch12. So it would be useful to know in advance whether 7.2 dBi would be "enough" before you cut and fabricate.....
So if the "SWR FIX" doesn't work, you MIGHT want to consider building a HIGHER Gain Yagi [if a narrower Beamwidth is suitable for your location], such as:
And if you eventually find THAT isn't quite enough....build a PAIR, stacked either Vertically or Horizontally, which MAY provide 10+ db of Space Diversity Combining Gain [depending on severity of Multipath Fading] in addition to nominal 3 dB Summing Gain.
If you decide which two Antennas you want to stack, I can calculate a HVH or HHH to interconnect them.
FYI: The only V-Stk Hi-VHF Pair I've Optimized thus far is a Pair of simple CIRCULAR LOOPS with Reflectors...which is also quite good [Screen Reflectors could be Aluminum Foil on Cardboard if in Attic....or Re-Optimized to use Reflector Rods]:
BTW: I've been doing some preliminary looks at alternatives for an FF4/FF6 with improved Hi-VHF Performance [at expense of UHF Performance], but thus far the only examples I have ready are as follows:
[All SAME, NOT FF]
[All SAME, NOT FF]
I'm going to go ahead and do some Optimization Runs with a combination of these ideas: FF4 (instead of M4) with two Rectangular Hi-VHF "Resonator" Loops....and then add the two LAYERS of Reflector Rods (same or similar to SBGH Reflector Rod's). PS: This was a Re-Optimized spin-off of alternatives that could be HIDDEN behind a user's 3-ft H x 2-ft W Metal Picture Frame....this fully Optimized alternative assumed two smaller Metal Picture Frames....