This paper describes a Q-band resonator that permits study of a sample large enough (3 mm o.d.) that the same sample can also be studied at lower frequencies (S and X).  This is an important contribution.

The authors should compare the special features of their resonator to the commercial large-volume Q-band pulse resonator.

The authors properly point out that the 3 mm sample diameter is a large fraction of wavelength at 35 GHz.  One consequence of this is that there will be a phase change in the microwave field in a dielectric body.  This is not discussed in the paper.  Was that phase change calculated or measured to be negligible for these samples?

The authors helpfully provide the experimental parameters used in data collection, but readers will benefit from explanation of the choices.  Some of the choices seem arbitrary.  For example, the differences in relaxation times would suggest using higher incident microwave power for DPPH than for N@C60, but the reverse is reported in the paper.  Why?

What guided the width of the slots cut in the resonator for penetration of the modulation field?  Sidabra et al. JMR 274 115 2017 discussed optimization of the slot size.  Was this result used?  Was the slot dimension chosen consistent with the design guidelines of Sidabra et al.?

The discussion of the resonator efficiency should be expanded.  Why is the efficiency so different for the two samples used in the calibration?  A measurement at room temperature would also aid the explanation.  One possibility is that the resonator Q was different because of the temperature dependence of the conductivity of copper metal, because the coupling changed with differential expansion as the temperature changed, and because the conductivity of coal lowered the Q relevant to that measurement.  The fragmentary information provided in the paper is not helpful except to stimulate questions.

The paper states that a 150 W pulse amplifier was available but that only low power was used in characterizing the resonator.  The summary paragraph describes the resonator as for “low power pulse EPR.”  Is this a statement that it can be used successfully with low power or that it is only useful for low power?  Are there places where arcing would occur if 150 W were used?  Other large Q-band resonators have focused on being able to perform DEER experiments with large samples and high-power amplifiers.  This paper should clarify the role of this resonator within this common application of Q-band EPR.

The discussion of CW EPR vs. field-swept-echo-detected could be usefully expanded.  Field-swept-echo-detected exhibit nuclear modulation that is dependent on time between pulses, and a field-dependence of echo phase memory that results in intensity dependence on field.  If anisotropy results in small slopes of the absorption line, the CW derivative spectrum can be near zero where the echo is large.  However, if a narrow line can be fully excited in the pulse experiment, exact quantitative agreement between CW and pulse spectra can be demonstrated.

Revisions in response to the above comments will make an important contribution more understandable.

The authors describe a 35 ghz EPR resonator with a high Q-value of 3300. The resonator is well designed and the drawings are given to the community for dissemination. However, it is not clear what advantages this resonator has over other fixed end section designs and the authors have missed several features of resonator design well documented in the literature to create an even more impressive resonator. 

Minor edits:

* page 2 line 25, remove the word "for" at the end of the sentence

* page 2 line 49, change "these two bands to Q- and W-band" for clarity

* page 5 line 155 has a reference to the figure missing in my version. 

* there is no advantage to quoting dB for attenuation, please just report microwave power. 

Major issues:

* The authors have missed a fantastic paper by Yuri Nesmelov who went through the sample size vs EPR signal dependence in a TE011. It is not clear on this paper that if one was trying to maximize CW EPR SNR would this be the value? it would be expected to try to maximize the EPR signal with sample tube geometries diameters. Is 3mm that point at Q-band with frozen samples? 

* Mett & Hyde investigated microwave leakage that is inherent in all TE011 cavities with modulation slots due to the coupling of a TE311 mode. Their findings was the need for a decoupling ring of the end plates which will make a more pure TE011, especially with large sample access as they published. Q-value of the system could be improved by reducing or removing this leakage that will absolutely exist with the size iris and large slots chosen for magnetic field modulation. 10.1063/1.1823748 There may be "no additional modes" but your simulations clearly show TE311 distortions. 

* From Fig 1 I was able to determine that the resonator geometry is 11 mm in diameter and 9 mm tall. It is well known that you will get maximum Q-value with a D/L of 1 due to the ratio of the stored energy to the wall losses. Further improvement would be expected from such a design. Why was the same geometry of Satvisky chosen if optimizing for CW? 

* There is no mention of spins or concentrations of any of the samples. it is not clear if this resonator is on par with other designs or better.

* Any resonator paper for CW should come with a power saturation curve. it allows one to see what the performance is for a saturating and non-saturating sample. This can be performed with free tempol or labeled T4L.

* What is the point of fig 2? The frequency will shift upwards as the sample tube is slowly removed from the cavity, and the profile of the magnetic field is cosinusoidal. Is there something I a missing? This is not a key finding and could be moved to SI. 

* According to your simulations and experiments, there is a "maximum q value" What does that mean? Is it critically coupled at +/-3 degrees and then overcoupled in the center? How is Q-value measured? It should be measured with -3 dB points (half power, -7dB in your case due to the -4 dB losses of the waveguide) while critically coupled (<-25 dB) and an unloaded Q value can be calculated as QL = Q0/(1+beta) where beta is 1 for critically coupled. Then you can measure VSWR at the over coupled positions and VSWR = 1/beta for under coupled, VSWR = 1 for critically coupled, and VSWR = beta for over coupled. Please report beta, and report what temperature and sample you had in the resonator when the Q-value was measured. This is not described in the methods section 2.2.

* Table 2 should also include the simulated values for the Q-value, measured and simulated beta coeff for overcoupling, calculated conversion factor, etc. 

* With the common use of cryogen free cooling systems, one needs to really worry about vibrations, especially with CW where the vibrations are on the order of the time of the experiment. It is not clear if the pendulum design of the coupler has any improvement over the movable short design of Reijerse or that of a rigid PEEK rod with pill. My intuition would say the pendulum might be problematic. Especially with a small capacitive iris with large stored energy as designed. No comments are made about why this is an improvement over other designs. 

* Filling factor is defined as the ratio of one part of the rotating component of B1 with only the components perpendicular to the static magnetic field within the sample over the stored energy in the whole cavity. See chapter 5 in Misra's book. I do not know how it is defined in Reijerse or others, but it is not correect as written in eq 3. 

* The measured conversion factor of 0.39 mT/sqW is fairly typical. And does not show a dramatic improvement over the Reijerse design or later designs by Satvisky here  10.1063/1.4788735 and here 10.1007/s00723-021-01404-4 

* Further improvements could be made by switching to tellurium copper. Tellurium copper is far better machined than the typical "gummy" pure coppers and has better temperature and electrical characteristics. Tellurium copper is also not susceptible to atmospheric corrosion (temperature cycling, etc) which will result in a very nice EPR background below 70 K, especially in CW.

* the authors say "in spite of an oversized sample geometry" Why would the oversized geometry potentially limit the linewidth? The resonator is of standard size and no B0 issues should occur with any lab sized magnet where Q-Band magnets optimized for a 15 mm cubic homogeneity "sweet spot". The resonator is made out of 99.995% copper, so little to no inhomogeneity from e.g. nickel. The field modulation was sufficiently low, and the nitrogen centers were most likely not saturating at the powers used. 

* In general, what can this resonator do that other cylindrical resonators cannot? I am not seeing any significant improvements.